Just about never. Maybe sometimes on GW style bases. But I only use those for figs I intend to use, in a game where the base size is important, like 40K. My Blood Bowl team is based on the 30mm Wyrd rounded lip bases, since the size doesn't reall y make a difference in the game. For the Wyrd style, I cut .010" thick circle out of styrene, and make an "insert" of sorts. I set the plastic circle in an old scrap base , and I build up the base with Milliput or Apoxie. (Talc the base first, so the putty won't stick.) Then when it has cured, I pop it off the base. If I need the figure to be integrated into the base somehow, I'll make just a thin blank with the putty, to get the form and so it fits the inset of the base. Then I'll remove it from the scrap base, add the figure (or more putty, and then the figure, depending on what I'm going for) then prime them together.