Melondar,
Most of these guys are nancies.
Inflate to build factories after infra five, to build colonies, TCs, governors and any necessary forts before five, always up to five percent inflation (during a building spree-don't bother trying to keep it there if you have an important tech milestone ahead).
As a rule, inflating to 5 (2, now, depending on the mod) in MP is an absolute must and it's a reasonably good idea to keep stab at 2 and drop it by one with DPs or DOWs when it's at 3, without bothering to reinvest. The reasons are simple:
1. Resources in MP, particularly early, are a lot more important than in SP. Sitting at 0 inflation, unless it's for a good reason, means losing A. The rewards of factories/colonies/merchants/whatever, B. Any deflation events and C. Any incidental deflation from Governors (it's more efficient to inflate in one shot than each month).
2. With stab, there's always some fairly substantial natural investment, depending on the country, period, factories, etc. Of course, it depends on your stabcosts, because 3 gives you a nice cushion, but generally it's worth clicking DPs and such and dropping to two.
3. Stab 3 confers only marginal benefits. Stab 2, on the other hands, provides an extra merchant, so it's the target.
4. There are a ton of positive stab events and deflations are fairly common as well. No, it's not a good idea to bank on these, so no crossing 5 unless there's a good reason or dropping below 2 stab with the expectation of a good event. But it's perfectly fine to develop those two small cushions, so no events go completely to waste.
In SP, if you like to play it to the hilt to see how well you can do or if the game is still difficult for you, all of those rules apply to a lesser degree. In fact, you can go ahead and inflate a lot more and risk lower stab and higher BB in SP. Even thirty or forty percent inflation with a major is nicely recoverable, mainly because the AI invests in tech so badly.