Doing a 'down and welded' check in a fixed grear can lead to trouble in a retract. You train yourself, while flying the 'fixed', to chant "down & welded" and do nothing (there's nothing to do). This is setting you up to do the same in the retract!
I disagree. When I converted to a complex retractable because I had been practicing the checks for retractable gear since I started flight training I had no problems remembering the gear check on downwind. In fact, it surprised me how natural it was. The prop setting OTOH is another story...
I do from time to time mix up the "down and welded" response to "gear is down, three bolts" when I fly fixed gear.
