Unfortunately, there can be a bit to worry about. No, neutered males cannot impregnate a bitch, but they can breed her. And sometimes, that can cause injury, either from the bitch not being receptive, therefore snapping at the male, or an inexperienced male being aggressive in the way he mounts her, or separates a tie. Inexperienced owners contribute to this by not knowing what to expect.
Mind you, the above scenarios are rare, but with dogs who have never been bred, and people who have never dealt with breeding dogs, it can lead to injury.
My suggestion would be to separate her from the male, whenever possible. At the very least, crate her or the boys...
I have never seen a bitch that is not receptive beeing mounted successfully by a male...
If your neutered males are VERY interested it's better to separate them for those few days...normally even intact
males don't care unless the bitch is really receptive...