For most job offers I've gotten I've asked to review the source code post-offer. I've even done this in a couple interviews -- can't recall ever being denied.

In one particular instance I came back to review the code for a product that required a level 3 security clearance; the hiring manager happily complied, letting me spend over 2 hours alone studying the code on a workstation in a side office. One reason I passed on that position was due to the code being more spaghetti than I would have preferred.