I want to do the Genocide route in the game (so I can do the Genocide only boss fights), but I also do not want to corrupt my save data with the effects of the Genocide route and get the 'soulless' True Pacifist ending. At what point can I still reset without affecting my save data?
(SPOILERS, MUCH SPOILERS)
You can quit the Genocide Route when you want before Mettaton NEO by not exhausting the kill counters before his fight (you will continue on the Genocide Route and fight him even if stop killing before him, but then you will quit the route), after that, as New Home doesn't has monsters (only Flowey will be telling you about his life), you can't quit the route from this point. However, you can still reset before you meet Asgore and kill him and Flowey. Then you will meet Chara, when you can't close the game except if you use a Task Manager, so if you don't know how to open it, don't meet Asgore or you MUST give choose between ERASE or DON'T.
P.S: If you end up having to give the response to Chara, don't choose DO NOT. You will regret it. Also you can remove the Genocide Route effects by using Undertale Genocide Remover, Flowey's Time Machine or erase it manually (If you're on Steam, it will mess you up).