I'm mostly with Mike on this one, with a variation.
For guitar parts, I listen to the part. If I hear something that sounds "off," I move the transient marker to the beginning of the actual note, as Sonar doesn't know whether the noise before the attack or the attack itself is the note.
Then I move the transient marker so the note starts in the right place. Typically there will be 6-12 or these or so in a part and then it's done.
I almost never snap an entire part unless it's something brainless, like a four-on-the-floor kick.