Nice to nail it, but sometimes you can't. If it's complex, I generally learn the bones of it then add detail, but I get a lot of satisfaction from playing something spot on, especially a walking line.
It also depends on genre. If it's the Faces or a Rory Gallagher song then why try and duplicate something the originals would have played differently every time themselves? Use it as a chance to develop my improvisation.
Or circumstances, doing a gig on Saturday, mostly new stuff and styles for me. Acoustic, fretless (me) and vocals. Very few rehearsals and some songs I've only played once so far... some songs are totally improvised, some are in the spirit of the original, a few are by the book. I've learned up to the start of the second verse of Wherever I Lay My Hat and beyond that I'm 'in the style of' - going to be hairy as it's just going to be bass and vocals (am I scared - no I'm s***g myself!)
*Ace of Spades - I've had to learn how to play bass chords, super fast, with a plectrum. I would never have attempted this and have now discovered the main risk is cramp!
OTOH, doing a battle of the bands with the main band the week after. Eight songs, six of them we're aiming to nail*, one I simplified but I'm now adding in bits and for one I'm going to just have as much fun as I want as we'll all go OTT 🙂