I do it all the time...chiefly because I rarely build the 'state of the art' kits that most P-E is designed for, and more often want to add detail to old Airfix, Frog or Revell kits and the like.
Generally there are very few fit problems except for instrument panels and cockpit consoles; there the forethought Bish mentioned is an absolute essential. Panels often need to be trimmed (or the areas where they fit 'enlarged' via judicious hacking or grinding away too-thick plastic walls with files or a Dremel). All of which is much easier to do at the start, rather than when exacting cockpit-detailing is finished and then you realize the blasted part won't fit!
[Ask me how I know that.... ]