You use compression when the dynamic range is too wide.

That's basically it.

Establishing exactly on what and when the dynamic range is too wide is the "art" part.

I recommend using it on bass, vox, drums, and clean guitar. distortion units will often compress the signal for you so there is no need to compress a guitar/bass with distortion. be carefull not to squash the signal too much. some people compress to tape and others wait untill mix down. I personally compress to tape.

hope this helps.

Here's where that whole "art" part comes in -

I'll fairly normally apply a bit to vocals *if* the vocalist needs it, but only on the rest (bass, acoustic or clean electric, drums, etc.) if it's absolutely neccesary.

Not that either way is right or wrong - Different strokes for different folks and all that.

Compressing to tape is another story - Again, if it's what the doctor ordered, that's all good. However, once it's there, it's there. If you're not absolutely certain (as I would assume by your orignal question), I'd suggest getting the mix "in the ballpark" before using ANY compression, then applying it to what is "asking" for it.

thnx guys
I appreciated it

much love

when it sounds better compressed.

THAT'S it.

I never use compression to control levels in some math or meter related way.
I use it because I like the sound.

I almost always use a bit, just a bit, of compression on most things... as they get recorded; with the exception of drums.
I might compress room mics and I will certainly compress the overall stereo mix, which will effect the drums as well.. but I never compress individual drums.
I just don;t like what it does.

But certainly all guitars and vocals and bass guitar get some compression.
And usually piano and organ as well.
Synths are a maybe.

Other instruments: brass, woodwinds, strings.. rarely.

but it's all a matter of individual taste.
not rules.

as little as possible, and so that you don't say "oh that sounds compressed" I really like to a/b alot when I'm compressing something.

unless you're going for something effecty, which I often will on bass tracks, and sometimes guitar.