My experience is that some films (particularly some Ilford films) come out slightly purple even when they've been very well fixed, cleared, and washed. If you're getting variable or splotchy results, though, this almost certainly indicates insufficient fixing and/or washing. Using hypo clear will help get rid of these dyes in many cases.
You can re-fix, re-clear, and re-wash your existing negatives to get rid of any problems in your already processed negatives. For new negatives, as Claire mentions, the rule of thumb is to fix the film for twice the clearing time -- however, I'll add that most sources say you should fix the film for three times the clearing time if you use T-grain films. After that, I use a short (~1-minute) rinse, a 2-minute hypo clear step, a 5-minute wash or (for films fixed with a non-hardening fixer) the Ilford washing sequence (fill the tank, agitate five times, dump; fill the tank, agitate ten times, dump; fill the tank, agitate twenty times, dump), and finally a 30-second dunk in Photo Flo.
When you're working with 35mm, getting a clearing time estimate is easy: You can use a small snip of film (such as from the leader) dunked in fixer. I'm not sure what to suggest for sheet film, since I've never used that and I don't know what sheet film users do to determine clearing time. Maybe you could sacrifice a sheet -- cut it up into small squares to use for clearing-time tests. Note that clearing time varies from one film to another, so you should determine clearing time using the film type you're processing.