In South Hold it's less of an issue because between me and my vassals, we ARE those provinces. Rowena and Kaeranne kick some tithing in along with me, and the temples receive some basic rights - mostly ones that reassure them of their own security - and support in keeping those provinces THEIRS. In provinces where things are more diverse, I could see it getting a lot more complicated.
Liborius's comments earlier, about paying for 'where', not 'if', actually bear heavily on my area. At the start of the game, Osoerde and Bellamie were on VERY touchy ground with each other. Meanwhile, Osoerde was negotiating with the IHH for blessings and friendship - I didn't know the terms, precisely, but I knew they included 'X provinces of Osoerde to be blessed each season." So, I arranged a treaty with the IHH that basically consisted of me slipping them a little more gold so that Algael and Brothendar were ALWAYS two of the provinces blessed - allowing Osoerde to essentially shoulder some of the cost of the blessings FOR me, and thumbing my nose at William quite vigorously! Not to mention maintaining the image of the IHH and South Hold, shoulder-to-shoulder, against William when needed.
These days, Tristan and William have patched things up to one degree or another, but my treaty with the IHH still holds. An example of agreements/laws outliving the circumstances that created them, and of how politics can inform the treaty as much as financials.
