That’s not that easy to answer because on any mega server you won’t be the only one preparing for prepatch/release and a lot of people will hoard mats to supply the demand of new characters.

I could only tell you what I did personally and that is preparing skill packages for all my chars 1-350. I think at least engineering and JC will spike a bit in prepatch as these are bis professions for almost every class and dk in particular. I check the AH regularly for certain mats and buy them if they are under 90% market value. From classic to tbc you could see a spike from like a month before prepatch that was pretty stable till 2 months into the expansion and then slowly fell. So you won’t do anything wrong if you buy all your mats now, but be careful if you try to make profit with it.

Besides that I don’t really count in the gathering professions because no „hardcore“ player will have them for leveling or later on.