There are only three good reasons not to get your rabbit spayed: Something about her makes the procedure risky (advanced age, illness, or medical condition), you plan to breed her (which, unless you're an actual rabbit breeder, DON'T), or you don't have the money (in which case, save until you do).
Unfixed rabbits are plagued by hormones. Though there are exceptions, on average they are more aggressive, territorial, bad-tempered, and really don't play well with other rabbits.
And the #1 reason to get her spayed: unspayed rabbits who do not breed can have up to an 80% chance of cancer in the reproduction system! This chance is largely determined by age, of course. But everything is more likely to come down with cancer with age. It comes with having lived longer. The House Rabbit Society helpfully summarized the study (yes! The scientific study!) that started this statistic:
"Greene monitored a colony of rabbits over a long period of time (8-10 years). All rabbits dying from whatever causes were necropsied. 16.7% of 849 rabbits were found to have uterine adenocarcinoma. The average age of the colony was greater than 4 years... The tumor incidence in Greene’s colony was very closely linked to age. In rabbits dying between 2 and 3 years of age the incidence of cancer was 4.2%. In rabbits 5-6 years of age the incidence is 79.1%"
To quote the House Rabbit Society:
"Some decisions about caring for your rabbit are difficult to make. This isn’t one of them. This procedure prevents or solves all the most common behavior problems facing rabbits who live with humans. It prevents or cures serious health problems, too. In addition to living longer, happier, and healthier lives, spayed and neutered rabbits do not contribute to the tragic overpopulation problem... Males can be neutered as soon as the testicles descend, usually between 10-16 weeks. Females should be 6 months old..."
And having owned three rabbits myself, two females (one who died at 7 years old unspayed, with a suspicious lump in her abdomen and the other spayed just last month) and a neutered male...
GET HER FIXED.
It is a riskier procedure than neutering. It's an open surgery, of course it's riskier. But according to one rabbit organization, an experienced vet should have a success rate of over 99%. My vet explained to me that the biggest complication for rabbit spays is when an inexperienced vet damages the intestines, which almost never heal properly in rabbits. For an experienced vet, rabbit spaying is the EASIEST kind of spaying, as all their reproductive organs are at the top and easily accessible.
DO RESEARCH. Rabbits are considered "exotic" pets, and most clinics won't take them. Apparently only dogs and cats are not considered exotic, and even the exotic vets sometimes will only neuter, and those that will spay can sometimes only get one or two rabbits a year. I created an enormous spreadsheet of over 20 different clinics in the surrounding area, I emailed all that had emails and called the rest, and interviewed them on if they take rabbits (13 did), if they do spaying/neutering for rabbits (only 7), and those that were left I asked them stuff like how many vets they have that could perform the procedure, how often each vet had performed it, how comfortable they were with it, how many incidents of patients passing away, what anaesthetics they use (certain anaesthetics are no good for rabbits, and there's only a few that are okay. This was a trick question I already knew the correct answer to), what they do to make them comfortable afterwards, if they wanted the rabbit to come in on an empty stomach (also a trick question, rabbits should never have an empty digestive tract)... I also kept track of which clinics were recommended by the other clinics, since the clinics that either did not take rabbits or would not perform a spay would say to me "sorry, but you could try such-and-such clinic..." and I also kept track of when they would also say why they were recommending it (I take my rabbit there, I've heard good things about them...) so I knew they were recommending it because they thought it was a good clinic, not because they'd just heard it took rabbits.
I narrowed it down to two clinics. Both were highly recommended, one was more impressive as they were the clinic for a zoo and therefore specialized in exotics, and they also had a very impressive aftercare. But they were also charging $400. The one I went with was highly recommended, had a vet very experienced with spaying, had done three successfully the month prior and therefore had very recent experience, and was just down the road from my house. They charged $250. (Keep in mind though that those prices are unusually high--I live close to a large city, which jacks up the prices.)
If you live in the US there are plenty of rabbit-friendly vet directories. I live in Canada so had to do the legwork myself.
And to address princessfional, the study I mentioned went on further to say that there were no cancer differences between genders in young rabbits, (4.2%) and it was only the females that the rate drastically increased with age. So no, cancer is not that much of a concern. Neutering is mostly a behaviour thing for males, fixing aggression, bad litter habits, and their complete inability to play nice with others. My neutered male is the most docile, agreeable, unaggressive rabbit I have ever met. In fact, when I was trying to bond the two unspayed females, he would literally mediate between them: If Delilah's boundless energy started getting on Picca's nerves too much, Lahi would draw her away and start playing with her. If Picca started to get too upset about Delilah's antics and attempts at usurping her dominance, Lahi would reassure her with cuddles and grooming.
