I definitely think it’s a good idea to check your bunnies each month for lumps and bumps — not just the belly, but the whole body. Even a spayed rabbits can get other types of bumps like abscesses or lumps not related to reproductive organs.
Most of the time I will find something new by accident as I hang with them every evening and pet them, so many times I think we will find new stuff that way, but along with the other regular monthly maintenance checks it’s a good idea to really feel your bunny from nose, jaw and the rest of the body. I have found bumps mostly by doing little circles throughout their body.
Jack had a little lump on his back/spine which was there for a month and then when I took him into the vet, we couldn’t find it! Later I found it again, but it was much less and by the next week it was gone. But if it hadn’t disappeared, I’d want to make sure to get it checked out— and it was pretty small, so I wouldn’t have found that through normal petting (I was doing t-touch which is based on a circular motion and that’s how I found it.) Bailey had little bumps on and in her ears they came and went as well that I kept close track of. (The vet had taken off one but it regrew – it was just a strange inflammation thing that was not dangerous or painful to her.)
So really a monthly check for new things is a good idea