I have always written complete IF's, but I've been using Synplify recently and it generates warnings about redundant code when I do that in a lot of places. In the past I have run into problems with incomplete IFs generating unintended results, so I just keep doing it the way I have-it won't cause any problems, the tools will eliminate redundant logic and I don't have to worry about it.
However, be aware that you may generate unintended latches if you don't write complete IFs.