Got a refurb iMac a few weeks ago. 0 dead/stuck/flashing pixels. But the service manuals, which you may be able to source, specify what is considered acceptable, and it's more than zero for the 2006 iMacs.
If it's essential to get something perfect, then you'll probably want to buy a pro-grade screen. If it's just because bad pixels are annoying (and, agreed, they are), you might prefer not to risk refurb and buy in-store or at least ask them if they have a public policy now.