I like 'em distant, at least until the PCs are so powerful that they have a chance to beat one in a fair fight. When the PCs are legitimate threats, then the arch-fiends might start taking them seriously.

So the PCs might see a Prince of Hell, but they wouldn't interact directly with one -- they'd meet subordinates (catspaws / handlers / cut-outs).

Old fiends are paranoid -- that's in part how they got to be old -- so this isn't just disdain. It's also a healthy dose of self-preservation.