Many of us like to compress our stereo mixes; the mix tends to come together more, be punchier, and have better levels. But if you’ll be delivering your mix to a mastering engineer, think twice before compressing. The mastering engineer will likely compress the mix (using a better compressor/limiter than most of us have in our studios) and if you’ve already added compression earlier, this may hinder him or her from doing their job – once a signal is compressed, there’s no uncompressing it!
If you feel compelled to compress your final stereo mixes before sending them off to mastering, do yourself a favor: also include an uncompressed version (that’s clearly labeled, of course). That way the mastering engineer has the option to work with whichever mix will provide the best final results.