Are postdocs in your own institution frowned upon? Is it typically preferable in academia to pursue postdocs in institutions other than the one one has received a PhD from? I have heard on the grapevine that this is the case, but is there anything to back up this assertion?
The extent to which it is frowned upon differs greatly by discipline and country. At least having been in different departments/universities is usually beneficial for young scholars. Departments differ quite a bit, so having been in different departments/universities/countries broadens the range of experiences. This is usually taken into account by hiring committees, but again, the extent to which that is the case differs substantially.