I was just trying to think, and I'm sure I'm cursing myself by even answering this question...

In all the times I've entered the US, by land or by air, with GE or NEXUS or without, I've never been secondarily screened. (At the Saskatchewan land crossings into ND and MT they often have you pop your trunk and have a quick look while they ask you questions, but I've never had them go further than that.)

I've never been secondarily screened entering Canada either, for that matter.

I suspect it's inevitable, but if you seem honest (and the best way to seem honest is to *be* honest) I suspect it'll be a rare occurrence.