Also, which of the 6 different artillery formations do people use? I tend to just use whatever is available, again typically three per army.
As previously posted, I'd say the 41b corps artillery and the (available from August) army artillery regiment are the most useful, because they use good guns (well, the 41b corps artillery regiment uses regular 152 howitzers, but it upgrades to the army artillery regiment and thus the ML-20). 12 extra 76mm's and 24 extra 122mm's that the Germans will probably damage or destroy with counterbattery fire are not too problematic for the Axis. However, the ML-20 has a much better range than the regular 105mm and 150mm German howitzers, so the danger of counterbattery fire is less.
OK, but is it not the case that 76's and 122's that are never used are by definition exactly as "not too problematic"?
I mean, artillery that is sitting in storage cannot possibly me more useful, can it?
You need to be careful with actually forming artillery units, as you only have an X amount of armament points to spend. Later in the game, you can probably disband most if not all support artillery regiments except for rocket units and ones using the ML-20 (because it remains a useful gun), because you have on-map artillery units.
So if I should not form units, then certainly using the units that are already formed is better than doing nothing with them, right?
I guess what I am trying to get at is forming the discussion more around choices, rather than just evaluating how useful units are in a vacuum. Are the 76/122 units so useless that they are better off just ignoring, and leaving your units without artillery (since forming new artillery is not a great idea necessarily either early in the war)?
Any reason to attach combat support to the Fronts? Construction units?
No in both cases, at least initially. Later on, you might want to attach some RR repair brigades to the fronts to repair rail lines (construction units attached to armies will generally also help out with building fortifications).
What about sappers? AT units?
I don't form/build AT units, I consider them to be a waste of resources. Sapper regiments are useful, you could park a number of them in STAVKA so they can train up and be assigned to corps later on. I'm reluctant to assign sapper regiments to army HQ's. I'd rather have only artillery in there in terms of combat support units.
Why? A waste of what resources - what is the key resource that needs to be saved such that one should not form AT units? I hate to belabor the obvious, but my guys are getting their asses kicked by German armor - surely some AT guns might help a little! And since I seem to have plenty of AP points, why should I not spend some of them on better AT defense?