I can respect Julia's reply, but I will counter it with: I've know many rats (hundreds) who have been neutered, and only 2 who died. On the flip-side, I've known many rats who have been spayed (again, hundreds), 5 died.
It's a risk, for sure. Any surgery is.
My opinion is: If you want a mixed colony, you must neuter (and/or spay). That's a given. If you have cage aggression, I highly recommend neutering. If you have an aggressive, rat that bites, neutering often (not always, but often) changes the rats (no more hormones can remove the problem completely).
Otherwise, it's your call. Many rat owners sputer all their rats with no problems. More so at other forums, but I know there's quite a few here who do as well.
If you do decide to go with it, make SURE you know your vet. Make sure they've done neuters before (and not just a few, either!). Make sure you find out what they use to put them under. Make sure you find out what they use to close the incision. Make SURE your vet is a good one when it comes to these things. Maybe even talk to all vets in the area and choose one that meets all the "requirements" of a good sputer vet. Even if you have to travel a bit further than you usually would. It's 100% worth the trouble.
All that said - mixed gender mischiefs are awesome.

There's something about the dynamics that makes for a great group.