Since I am of the belief that even adults should moist wipe post poop, we keep wipes on her toilet and I have her bend over to make sure she is good to go. She is pretty good at wiping herself but sometimes better than other times.
We just switched to toilet safe wipes. She recently started trying on her own and for a week or so I checked it after (by wiping again). She did fine so I let her do it herself unless she asks.
Re: Wiping of the butt