The thing is, just like writing is the only perfect practice for writing, the only thing writing flash fiction is perfect practice for is writing of flash fiction. For writing other kinds of fiction, it may teach you bad habits.
May not, to be sure. Depends on your weak points. For instance, particularly if you stick to a hard word limit, it teaches a lot about making words jump though hoops. On the other hand, for a flash fiction work, you want to pare an idea down to its absolute essence with nothing else adhering to it. Longer works, you need to have ideas that stick together, and build up a structure, if only to keep your readers from dying of boredom. Likewise setting and character -- if you don't make it a vignette instead -- have to be parsed down to a minimum and often implied rather than stated.