I can not disagree here. It depends on each situation, what and why one is pruning and how much is being removed to a large degree. If first time on the site, and only minor pruning is necessary (as in case of one we are doing this week - four small limbs on one tree - and few minor trims on rest - then it really is a matter of working with customer and then creating an inventory of site - recording over time - documenting and then going forward. (previous tree worker they had was single person who is now retired - but did a good job over time - so only minor items to pick up there)
But you are correct in that ANY removal of foliage at all can impact health of tree in question. Now if one were removing 15-20% of total foliage then one needs to consider a lot more. Guess it is what defines a trim to a large degree, species of tree, location, time of year, etc.
Again however, this can all come into play by on the job experience and not necessarily by certification. (You yourself said that you do not have after 28 years - yet you instinctively take items into consideration when doing work or discussing with client each day)
Some of it also comes into terms used locally.. when we use the word trim - we mean typically a fairly minor pruning job.
An arborist knows what a tree needs. (or don't need). We try to educate those who feel like they are paying "by the pound". There are classes of prunes, ie; crown clean , crown thin, structure prune, crown reduction, crown raise, etc..., there are no terms (professionally), such as, Lace, Balance, (shape), etc.., . For example, We don't prune more than 15% on pines and wait till late fall to do that. Mature trees re-act different than young trees. Knowing how and when to prune palms helps a good lot.
Jeff