First do what tabun said!
But you don't have to care about the grid at all when aligning textures.
If you need to align a texture on a brush just simply select the face on the brush which you are going to align then press the S key this will bring up the Surface Inspector there you can do alot of stuff, shrinking or increasing the texture, rotate it and in your case align the texture!
You have two options called Horizontal shift and Vertical shift manupulating those numbers will "move" the texture over the face. If you use a newer version of GTK radiant the surface inspector is all diffrent but i think you can find out what is what
Also if you want to align a texture on a curve brush hit SHIFT+S instead this will bring up the Patch Inspector instead, tho aligning curve brushes can be a bitch!