Flypaper textures are probably the first I bought rather than just downloading free ones. Initially, I think I was using textures mostly as backgrounds for leaves and flowers, so I chose the packs that had textures that looked good by themselves. Now that I'm using textures in combinations and blending them with other layers, that approach may not make the most sense. However, as I'm sure you know, blending textures with other layers can be a trial & error process. I've learned to make choices based on patterns, colors, and border effects, so that's part of the current process of deciding whether to buy textures, or not. The other part, though, is the same as when I started out.
I may have mentioned previously that one of the things on my to-do list is to go through all the textures I have and build a database so I can apply filters to pick out the ones I think are most likely to work with an image. Perhaps I should initially populate the database with just a few textures to see how well it actually works.