I don't reccommend using Auto Taxing if you want to maximise your income.

Auto Taxing will make taxes as high as possible without making the risk of rebellion any higher than 0%. England is a fairly secure territory, especially once the French get tied up in war with the Germans. It's a risk, but if you can raise taxes to the next level in a province and only have a 5% chance of rebellion, you might want to consider it, depending on how much you earn from taking such a small risk.

Another thing with auto taxing is that you lose track of what your tax rates are. Having to check with the shift key every turn keeps you on top of things; you might consider putting the tax to maximum in a rich province and maintain loyalty with bigger garrissons. For example if raising the tax two levels earns you 100 florins more, but you have to add another 200 peasants to the garrisson, then it's worth it (maintenance of the extra peasants is less than the 100 florins extra profit).

Paying such detailed attention to finance is boring, and the extra earning may be small but to me auto-taxing is like auto-resolving battles. All it takes is patience. Of course once your trade routes open up...