AngieA:
I do not recall any rules against using texture overlays either in this forum or in its previous incarnation at IFX. Many professional artists (including many that were featured in IFX Magazine) used texture overlays and even provided guidance in tutorials. However, just from an artistic perspective, texture overlays should be used sparingly and not overpower the painted art. Overlays are a good way to "punch up" certain areas of a painting and quickly add some interest and detail, but should be used selectively and with restraint.
I don't know if you subscribe to IFX Magazine, but in the August 2014 (page 86) there is a walk-through by artist Ed Binkley on painting Marley's ghost that uses a very prominent texture overlay for the jacket (most uses are much more subtle). I'm sure folks are divided over the use of textures, but their use is pretty standard in the illustration industry.
Here's a personal example on a piece I'm working on now called "The Soul Collector." The background is a painted base with textures brushes overlaid with a subtle concrete texture which was erased in certain areas and then with some more brush work above the texture.