Optimal texture creation techniques

I'd like to create textures that are custom fitted to specific SL objects. However, as there is no in-game texture editor, I have to create my textures in a separate program, such as the Gimp or Photoshop. (FWIW, I use the former.)

Unfortunately the external program doesn't have access to the particular prim that will ultimately receive the texture. Therefore it's a challenge to create a texture with the precise dimensions for final fitting.

What techniques do veteran texture creators use to get round this problem?

For flat surfaces, I put a "grid" texture I created on the prim, align the camera with the surface in question, and take a screenshot. I use measurements of that screenshot as the basis for creating the final texture. I don't use the grid for any measurements, just as a check for anything funny going on with UV mapping.

I haven't worked out a technique for curved surfaces, but I expect I can use the grid for that as well.