Binding textures in OpenGL

The answer is difficult, because any strong GL implementation should make binding a already bound texture a no-op as an optimization. You should try benchmarking to see if putting a condition makes a difference or not.

The same applies to unused textures in texture units, since the GL implementation does know what texture units are finally used, it should not affect performance (as an optimization) to have unused texture units.

The holy rule of GPU programming is that less you talk to GPU, the more she loves you. glBindTexture is much more expensive than single condition check on CPU, so for frequent state changes, I think you should see if there is a need to transfer new texture index to GPU.