It's not something we have done before, as I've never seen it happen. I pressume your talking about the fronts? What E46 do you have? The M3's come with a strut bar that's supposed to stop this from happening.
I've got a 2001 330i Touring. From what Ive been reading it seems a sensible precaution when running coilovers, might as well chuck them on at the same time as the coilies. Front for sure but I think they are available for the rear as well, I believe they are part of the 'heavy duty' option. Im think I read that M3s come with them as standard.
BMW part number 51717036781 for the front. Not sure about the rear though, cant find any info about them right now. I guess I can go get them myself from BMW but if you do them then its less hassle. If its going to be grief for you though dont worry about it.
Rears seem to be a Z3 part, found some copies on ebay but in the US. BMW shock mount reinforcement plates | eBay
The E36 M3 3.2 had reinforcement plates, and I know the topmounts can be swapped between e36 and e46. I've seen a couple of "mushrooms", they don't seem to be very common, but they do have issues every now and again.
