Forgive me, our posts are overlapping so I am a little behind.
It is doubtful that the starting load of a number of residential garage motors will be acting in unison. In a commercial application you could have workers numbering anywhere from 2 to 5 to infinity but in the typical home shop you have one or two. Again, you need to do a load calc to have any idea what you will need and #2 Cu may be a better choice than #3 for some garages (especially ones 175' away!). One thing you need to realize is that some motors will blink the lights no matter what. Unless you have wired right to the ragged edge, using a #2 over a #3 may not help. The home panel may be overloaded or underwired. I have been called out to homes (wired by others) that experience a slight blinking of the lights when the a/c kicked in. All connections can be tight, relay contacts can be good, the load on the panel can be ESTREMELY small, the wire feeding the panel and the a/c can be large in diameter and short in distance but the problem persists. In those cases, a new a/c or a larger lateral from the power company MAY have solved the problem. The point is, oversizing a subpanel feeder is not a cure-all. A larger wire provided by the electrician does not somehow act as a capacitor for an extra burst of energy when needed (I realize you know this). Voltage drop in the subpanel feed is just one of many possible causes and may not factor in at all.
You have to draw a line somewhere and use some math skills rather than making a shot in the dark and hoping for the best. If you talk your customer into paying for a wire that is 1 or 2 or 3 sizes larger than another contractor quoted, and his lights blink anyway when the job is done, what is he going to think about your competence then. Without data to back-up your claims then you have nothing to fall back on. Like I said, from my experience, some loads will blink the lights no matter what actions you take (do a search here on a/c loads for verification).
...all those words and I don't think I have answered your question yet iwire...
Of course you need to add all the continuous and noncontinous loads together (here I am using the code definition of those words). The starting load of a motor is not a continuous or a noncontinuous load for use in calculations. For motors you need to use the full load current found in the codebook. No pc program can take into account: the condition of the motors involved, the load already on the main panel, how well that main panel was wired, etc..
I notice that the more I write, the more I ramble and the harder I am to understand. Thus I will sum up all my words in one sentence.
You can not assume a proper size panel without load calculations much less sizing the wires that feed it.
