Motion blur is relative to the rate which the image travels across the film plane over the duration of exposure. Because movement is alwyas inevitable, even with a tripod, a "sharp" image is one where the image travels a distance shorter than the resolving power of the imaging system. Thus, magnification is an important aspect to determining if you need additional support or image stabilization. While 1/250 might be a good minimum for 100mm, you could easily get away with 1/100 with 50mm, and even 1/50 with a very wide lens, provided that the subject is not moving very quickly, or you are positioned very close. Likewise 1/250 would not be suitable for 500mm, even focused to infinity. I think the question you should be asking yourself is if your images are not as sharp as they could be, if the answer is no, then you'll need to be thinking about how to better stabilize the image, either with a tripod, image stabilization or faster shutter speeds.