it depends. USUALLY and MOST OF THE TIME you can just drop the app bundle to thrash and be done with it, but some specialized apps (such that need low-level hardware access, for example protools and final cut pro) in fact install files to /System and /Library directories, too. and most apps also write user preferences to each user's own library folder.
now if you just drop these specialized apps to trash, leaving those few files behind, usually you only lose some disk space. that's not so important to find all these files and delete them, UNLESS those apps have decided to run some background processes which keep consuming resources even after the app itself has been trashed. you can find those processes with activity monitor, should there be one. usually there isn't but sometimes there is. such a background process will have to be killed manually and the file has to be located and deleted to prevent launching of such background process in the future. also, the launch command of such process has to be written to some other preference file but finding that can be tricky.
don't be afraid, it is very rare to find such an app that leaves background processes behind, but just in case you find one, that needs special attention.
as far as the user preferences that the deleted app has written, well, you can just forget those. user preference file is a text file of a very small size, and deleting that can actually be more harmful than helpful - i mean, that kind of deleting will fragment your hard drive eventually. it is just better to leave it there. if you think that your user library has become too bloated, it is better to make a new user and delete the old one altogether (or, to have a special "shareware user" account in the system just for testing new apps and let it bloat instead of your actual user.
but as a rule of thumb, you can just trash the app bundle.
