In general, the more overlap the better. But of course like with everything, the trick is to find the least amount of overlap with the best results. Its true with photo panos, and more true with renders. With photos I like to overlap by about half or a little less. But that would be a lot of rendering in TG. Photos are of course a lot less time consuming than rendering, as everyone one here knows.
Since you are using free ware, I doubt that you will want to pay for a pano soft, but "Auto Pano Pro" is the soft I like. And Even with a paid full version of TG, there are still lots of creative reasons you would want to use stitching software. There are a lot of classic photography techniques you can apply to TG, just as you would to an image created with a real DSLR.
Its probably true of lots of 3D soft, but I find it really clear and understandable with TG. I have more problems with other 3D soft... In terms of treating renders as photos or using photography methods in or on a render.