I like to use water that is about room temp. I don't know that it makes any difference. As far as splitting the stems. I used to do it, but don't now. Not because it didn't work, but more because that it seemed to not make much difference. so, why go to the trouble. Now I just do like Hush--cut at a 45 underwater and scrape the stem.
Am umbra mentioned, cleanliness is critical. Be sure to sterilize your razor blade and make sure everything is clean, clean, clean.