A LOOSE "rule of thumb" would be to gently limit peaks on the way in (if you need to) and compress after.
The only reason you wouldn't want to regularly compress on the way to tape (?!?) is that once it's there, your stuck with it. The more practical experience you get with it, the more you're able to get away with.
Gentle limiting does (essentially) nothing to everything that's NOT a peak. So, applying more "traditional" compression later can still be done with a minimum of compressor setting "battles" going on.
Honestly when recording to digital you shouldn't be going over -6db ever. I'd set a compressor to start kicking in around -8db with a 2 or 3:1 ratio rather than strictly hard limit. You may go a bit over -6db but you won't clip unless you really give it some juice.
When the ratio is above 10:1 it's considered limiting instead of compressing. Infinity to one "in theory" should be a brick wall. Don't trust that wall - Always leave a little headroom. If you want to limit the peaks, you're looking at 2-4dB of limiting tops. Anything more drastic than that is another issue.
although i never compress on the way in... i pretty much agree with cloneboy except that i don't start to worry until i get to about -3db consistently.