I've edited one character once and I could play online no problem. I think it depends on how you edit the save. Powersaves I think I heard prevents you from playing online, where as APM save editor you have to decrypt the save to edit it and then re-encrypt it.
I've since deleted the character I edited, kinda went a little crazy with it and kinda took the fun out of it. If you wanna play online sometime, lets exchange FCs. I have a new character and am trying to get my HR back up.
EDIT: I reread OP and I think it really depends on how you use it. I would use it to give myself the material I needed to craft something, never changed the values of the armor or weapons or made nonlegit talismans. So maybe doing that could keep people from playing online, but when I have, I haven't noticed anyone or seen anything too crazy.