It seems there are a lot of debates regarding oils and powders.
Personally I think if you are going somewhere where you may really, really need it then, of course, it may be wise to. I agree, but I also think oils and powders shouldn't just been used like you are a gillionaire.
I think the purpose for the oils and powders are for emergencies so that way you don't screw up the party and yourself in the long run.
Whats preferable? Waste oils and powders from a starting safe zone and end up not having enough when you get into a tight jam just because of no start up sneak/invis from magic? Or...when you are in a safe zone, if its cool with the mage, "ask kindly and courtiously w/o demanding one" for sneak/invis, and if you "happen" to fall into a danger spot and the mage is not nearby "use the oil/powder" and save your life and possibily the mages as well?
I dunno..I just think considering the amount of oils/powders you can only stack, its best to go through an aggro field wisely rather than being too depending on the mage or not coordinating witht he mage and the stock you have.
So for people in places like that, think about it beforehand:
-Rely on mage too much?
-Coordinate with the mage and also have oils/powders on you to prevent danger to the pt and yourself?
-Waste oils/powders freely like a mad man which can nib you in the rear and your pocket change?
Personally i'd go with the second choice. Keep you and your pt safe, and there should be no worries. If a mage however offers to back you up thats at their discretion.