My promotion philosophy is generally inline with my tactical manoeuvre on the battlefield. The first big question is whether to go for Shock or Drill. Which depends mostly on the terrain you will be fighting over. Considering that rough terrain has the highest defensive bonus this ought to be the most highly contested ground on the battlefield. The unit positioned on forest/plains/hill will have a significant tactical advantage over one on floodplains/grassland/marsh.
Another point worth considering is that melee units will occupy the ground of any enemies they kill. So it is often better to prioritise staying on high ground vs. making a kill - unless the possibility of reprisal is eliminated. Given these tactics, my melee units will be spending most of their time taking and holding the rough ground. So I give them upgrades to match.
Ranged units, and cavalry, on the other hand need to be given upgrades based on where the enemy is located. Bombardment can be good for negating rough terrain bonuses, but if your opponent takes the low ground, Accuracy can totally rip them apart.
My general upgrade paths are as follows:
Standard Melee - Drill I, II, and III, March, Shock I, Woodsman, Cover I, and II.
Assault Melee - Shock I, and II, Siege, Amphibious, Cover I, and II, and Shock III.
General Ranged - 50% (Accuracy I, II and III) and 50% (Barrage I, II, and III), Logistics, Volley, Siege, and Range (as the need dictates).
Medic AA - Shock I, and II, Medic, I and II, Cover I and II, Shock III.
Standard Cavalry - Shock I, and II, Charge, Shock III, Blitz, and March.
Recon Cavalry - Drill I and II, Sentry, Charge, Drill III, and Mobility.
Destroyers - Targeting I, Sentry, Targeting II, and III, Logistics, Supply, and Range.
Battleships - Bombard I, II, and III, Logistics, Targeting I, and Supply.
Land Interceptor - Interception I, and II, Sortie, Interception III, Ambush I, and II, Repair.
Naval Multi-Role - Interception I, Accuracy I, and II, Repair, Interception II, and III, Sortie.
Bombers - Upgrade as per target.
Notes
1. With my melee units, I will spend that extra promotion point to get a Drill III/Shock I. Because, the versatility is often good.
2. Destroyers should ALWAYS get Sentry before Targeting II. Their job is to find subs.
3. I always spend the 3rd promotion, on cavalry, to get charge. Because, that's how I use cavalry.
4. I only use the 'recon cavalry' sequence when I have the need for it. Using cavalry on rough ground is treacherous, but, sometimes you need to do it.