Two reasons to compress on the way in: signal to noise ratio and color. If your recording chain and environment is giving you the sound you want without a compressor, don't use it. Depending on the performers ability, the quality/properties of the equipment and environment to produce a consistent performance and the actual sound you are going for it may be best to go through a comp, though usually as stated at very low ratios (typically less than 2.5-3/1). As for color-the Beatles and every other recording prior to what, 1982 ish was done to tape which A: general limiting was the rule so as not to overload the tape (causing crosstalk and bleed from one track to that next to it , etc) and B: the tape itself was compressing the signal due to the nature of the medium. I tend to love the sound of tape when recording drums or bass and I find I can get closer to that, which is what sounds best to me, by running through either a transformer(direct box or some mic pre's) or some(not all) analog compressors. As previously stated I prefer the sound of a bass recorded through an analog compressor. Vocals, however, it's as I said, depends on the performer, equipment and room.