As I get to know my process better, the latest epiphany I've had is that, with my current workflow and equipment, it pays off to split the "rehearsal" concept into at least two different kinds:
1) Rehearsal for arrangements: once reached the demo stage (which I described in an earlier post, "demo" defined as song-can-be-heard-from-beginning-to-end), in this rehearsal I play each of the instruments time after time figuring out ways to flesh out things, adding variations, inviting serendipity to add findings...
2) Rehearsal for recording: once the arrangements have been fully decided, this rehearsal is about building the muscle memory necessary so that you can record the part in the least numbers of takes possible (because I hate editing; in an ideal world no creative person should be punished with this task)
This division helps you being more efficient because in each of those stages the focus is different, so they benefit from a different approach. When you are rehearsing for arrangements, goofing off is not only not a problem, it is almost a requirement. You try everything you can think of, test sonorities, do things just because... Accuracy at this point is not an issue, you're just exploring posibilities. A good analogy for me is seeing it as the rehearsals I used to have with my band; we played and played the songs and they got polished and "grew" by practice. Of course we didn't aspire to make them sound perfect at that point. We were open to discovery, and we had a blast with the process, too.
Here's a wonderful example of serendipity that I've experienced at this stage with one song: while recording a mockup guitar, just to test sonorities, I didn't make it to the second verse in time, and couldn't start strumming until a few bars later than where it was supposed. Well, guess what? That sonority, guitar muted and only voice singing over the rythm section during the second verse, turned out to give the song a change of vibe that works great; so the arrangmenet stays. I don't think I would come with that idea by myself. This is that 5% that always comes of "happy accidents" that I always love of making music.
For that same reason (you don't need accuracy), in this arrangements stage you can test all the effects and plugins that you want even if the latency makes the performance more "slimey"; doesn't matter at this point, you are testing the sonorities.
And then, once the arrangements are mostly decided and written down, then you move to rehearse for recording. There you aim for precision. In my case, it will imply recording the guitar via DI -so you get hardware monitoring through the soundcard, and therefore no latency-, and reamping the recording later. A bit unnatural, but the result is going to be tight as hell, so it's well worth it.
I'm in this stage right now, rehearsing a lot so that I don't generate a lot of "almost good" takes; like I say, I prefer this kind of repetition to the endless effort over soundwave forms, zooming, cutting and pasting, dragging... your mileage may vary.