It really depends on how much I will be using, and that will depend on my class.

Cloths for example, tailors will want 99 stacks, but other crafting classes only need it every now and then so stacks of 3~5 is best (and you definitely want to charge a premium for smaller stacks).

As to how much premium, well, the more useless the item, the higher the premium you can charge.

Useful items sell quick, so I can easily buy a stack of 99 at cheaper price and break them down into smaller stacks and resale at a premium myself. Useless items that I only ever need 1 or 2 for, then I will pay twice the unit price just to avoid getting stuck with stock I can't sell.

It's safe to go 99 if it's a basic material like grass, ores, logs, but even for these, you will always want to put up a few small stacks at a premium, because there will always be crafters just short of a few pieces to get their next level, as well as crafters from other classes that need a few of something they can't make, and they will buy at a premium.