When "pushing" film, yes, you would have to increase the development time. The reason being, you are, in effect under-exposing the film. In the reverse case, they might not alter the development time, since they are saying the film is not as fast as advertised.
Pop Photo columnist Tim Fitzharris seems to regularly shoot Velvia 50 at ISO 40. This may be his preference, as I know of no scientific data or other peoples' comments that verify the change. Or maybe other people just don't say so!!
Color print film has a tendency to 'like' over-exposure. Generally, a full stop extra is within what the film can handle. In other words, cut the ISO in half.
Back in the days of predominantly black & white, there was a "rule of thumb" to expose for the shadows and develop for the highlights. This usually meant over-exposing, then under-developing. But b/w film is a different animal than color, especially slide film, so you have to watch that you don't go so far as to affect color rendition. Those who have their film done at labs can usually give instructions for development according to altered ISO.
