I personally try to add as much as possible to the first production. Main reason being I rarely get to play at all and when I do I like to play as much as possible. I'll say I agree with Dave above about production being an art in itself. The current song I'm working on I spent about an hour tops doing the music (i.e. guitar, rhythm guitar, bass and vocals.) I've spent about 12 hours trying to get it to sound good. What really sucks is I STILL haven't made it sound good.
On the other hand, if your actually talking about just playing one guitar by yourself and singing, I keep it about as simple as I can only adding a riff here and there. I will say whenever I hear a band play a popular song and it doesn't have all those little riffs and parts in it that I'm expecting, I always feel like they didn't do enough and I miss those parts. Which is reason #2 I try to add as much in as possible. The little things.