As it is, top-level imports IMHO are only meant to be used for modules required to be used in the startup, everything else should be a local import -- getting everyone convinced of that is the main issue though as it really goes against the regular coding of most Python modules (but the time saved to start up apps I work on does definitely make it worth it).
I guess you meant "run all imports at startup is desirable to check if they work", but I have a hard time agreeing with that (personally I think having a good test suite is needed, whereas running more code at startup is not wanted).