It depends on the song.
If I am recording something as a solo acoustic guitar piece, I usually lay down a click track or a dummy rhythm track (with a bass drum on beats 1 through 4 and a hi hat on 2 and 4) and then record against it, especially if it is a medium or slower tempoed number. Keeps me from rushing as I do have a tendancy to speed up as the adrenaline starts pumping, and allows me to focus more on the beat and keeping time, then what I'm playing.

I'm get so obsessed sometimes with trying to make perfect sounding and executed music sometimes that all I notice are the string buzzes or the gaffs / mistakes, that chances are no one other than myself would even hear or notice. And then it's hitting stop and rewinding and dealing with this over and over for a couple of hours.

Having a click or rhythm track to direct my focus towards, takes the pressure off the music. When I'm not locked in and concentrating on the music and instead am focused on the click track and staying in time with it, I play better and the music takes care of itself. (Course getting up and having a wee toke for strictly medicinal purposes sometimes helps as well, as I can worry less about what I'm playing or how well I'm playing it, and just let it flow.)

Some things are best left to the control of the subconscience, especially where making music is concerned. If you know a song well, and can play it "in your sleep" you should try and record it that way, rather than trying to analyze and critique every note you're playing as you're playing it. Now if only I could heed my own advice. **G** Ironically, things always sound better to me upon playback than when I'm actually sitting there recording. Headphones have a way of amplifying your mistakes or what you think were mistakes. Whip the headphones off, and listen to it through regular stereo speakers and suddenly it doesn't sound half bad.