Switching is not in itself necessary after you reach a certain level. As others have mentionned, it's more a matter of whether or not you need the extra low notes and the availabilty of more notes in one position.
One thing to consider, in addition to what eriky4003 said, is that right-hand muting becomes quite important when you go beyond a 4-string. Anchoring your thumb on the pickup won't cut it anymore, as you'll get more annoying and unwanted ringing harmonics from adjacent strings. You might have to change your right hand technique to a "floating" approach rather than an "anchored" one if that's what you're currently using on your 4-string.