Our approach is to increase the revision only if it is a product change. When it is a change in the model, for example regarding formulas/conventions/ reference points on the background, we do not want to increase the revision.
That being said, Windchill is designed to support best practices. Release means you should lock it to prevent changes, revise to make changes. Use change objects or promotion requests to re-release. I've seen in my years 3 types of changes but you need to develop and document your own for you company. It comes does to risk, production process and it work case, what if sh$t hits the fan analysis.
Major or FFF or Non-Interchangeable modification: Change Part number (which if involves changing related CAD object numbers, ok). "Does modification cause part to be non-interchangeable with respect to its next higher assembly?" If yes, move up a level and repeat until you answer no and stop at revision change.
Minor, or non-FFF or interchangeable modification: Revision change which should include CAD IMHO but practices vary about keeping CAD in revision sync with Parts. My test I use with folks is put all the inventory parts in a bit, those in the current revision and those with the new proposed modification. Shake bin, close eyes and pick one at random. If any and all with work in their next higher assembly or end use, congrats, its a rev change.
Administrative, non-technical, non-critical attribute, title block: This are changes so minor that they are more cleanup. When faces with the overhead of processing a change, you would opt not to do it. Things like adding a simplified rep, datum, attribute cleanup, mass props calc. Most of these are CAD level but we take the approach of "mother may I" where we unlock for someone and immediate relock. These are treated like exceptions. Its important to not abuse this and should separate people (don't give admin to those whose this is this their day jobs).