I just plucked this from the Beer Blog by BeerSmith. http://beersmith.com/blog/2009/07/16/mashing-for-all-grain-beer-brewing/
Here’s a summary of the major enzyme groups found naturally in malted barley and their active range:
Phytase (86-126 F) – Lowers the pH of the mash. Lowering the mash pH has a number of benefits, though a Phytase rest is rarely used by modern brewers.
Debranching (95-112 F) – Helps to increase the solubility of starches resulting in increased extraction for certain malts.
Beta Glucanese (95-113F) – Breaks down the gummy heavy starches, which can help improve stability and extraction, particularly for mashes high in proteins and adjuncts such as wheat.
Pepidase (113-131F) – Produces free amino nitrogen, which can aid in fermentation.
Beta Amylase (131-150F) – Produces maltose, the main sugar fermented in beer.
Alpha Amylase (154-162F) – Produces a variety of sugars, including maltose and also some unfermentable sugars. Mashing at the higher end of this range produces more unfermentables and therefore more body in the finished beer.
But it doesn't sound like any of these rests are bad?