Yes, a good gun-belt is absolutely necessary with any type of OWB carry.
To answer the initial question, this may not be as critical an issue with IWB depending on the type of holster. With an IWB style with a single loop like Rosen's ARG or Alessi's GWH, or any of the Summer Special styles with snap-loops mounted directly to the front holster body, a good belt is necessary. The spaced front and rear loop styles (Sparks VMII, my In-Cognito) are more forgiving of the type of belt worn, so a double thickness gun belt may not be as critical with them. (A good customer of mine wears a G19 in one of my In-Cognitos with a braided leather Dockers belt with no complaints, except from me cringing every time I see it!) :smile:
Another factor to consider is the actual suport and stability of the waistband itself. With good jeans you can get away with a lesser belt for IWB carry because the waistband will support some of the weight. With lighter weight pants (khakis, Dockers) a good belt will be more critical, and with elastic waistband pants (Royal Robbins 5-11 series) it will be essential.
Something else to keep in mind, OWB or IWB, is that the holster slots or loops and belt width should match. Do not expect optimal performance from the best holster and belt if you are mating a 1 1/4 inch belt to 1 3/4 inch holster slots or loops.
Hope this helps...