Personallly I do not think that a tissue fitting gives you a proper feeling about what to expect in the final garment since tissue does not drape. But of course it is better than no pretesting at all. What I like to do in a semi or close fitting garment is to make up what I call a fast and dirty muslin (Ron and I did a webtv show on how to do this). I pretest the pattern in a fabirc which drapes and has the approximate weight of the final fabric. For example I will pretest a knit in an inexpensive knit or something in my stash that has been there over 2 years. For a coat I will pretest it in an old bedspread. For a fitted jacket< I will pretest in a heavy weight muslin. That being said, I don't ALWAYS PRETESTbecause I like you have limited time and just want to hope for the best. I hope this helps!
Sandra