As a general rule, the most armour I'll give to any unit is usually +2 (unless their armour rating already starts out higher, of course), maybe +3. This is especially true of light cavalry, as I like them to be able to remain mobile without tiring in the first minute of battle. (Besides, in my opinion, light cav shouldn't attack anything where it would need a higher armour bonus than that anyway. )

Of course, as 3 of my favorite factions are the Spanish, Egyptians, and Byzantines, I'm used to fighting in the desert a lot, and I tend to play to each of those factions' strengths. So since a lot of Byz of units start out with pretty high armour bonuses (katanks, VG, PA, Byz Inf. Byz Lancers, etc.), I usually don't need to go in for a lot of armour upgrades, if at all. Since most Egyptian units are built for speed and/or stamina, I'll give them just 2-3 armour upgrades.

The Spanish are a little trickier, as I have them fighting a lot of desert battles (against the Almos and later on the Egyptians). Since they're a Catholic faction, however, their units are built for strength and power--and therefore you generally want to give them more armour bonuses to augment this.

In the end, what I usually wind up doing with most factions is have half my provinces training units with a lot of armour bonuses (usually heavy infantry and spear/seargent/pike units, as they generally don't move around a whole lot), and the other half training units with just a couple armour upgrades (usually missile units, cavarly, and any fast infantry with a high attack/defense ratio, such as Ghazi's or Highland Clansmen).