Is it good practice to wave following cars through when safe? When I am being followed by a car that is unable to safely overtake, I would normally take the lane and adopt primary position as recommended, making an obvious move to secondary position once it is safe for them to overtake.

Sometimes the car doesn't immediately try to overtake and may miss the opportunity to get past me, even though it is legal and, in the cyclist's opinion, safe to do so. If I can see further around the corner (being further ahead) that the road is clear, should I wave them through, or leave the decision up to them?

I'm interested in safety and legal issues as well as avoiding irritating motorists for the good of cyclists generally.
Never. I drive a school bus (and ride a bike, of course - but never at the same time) and when bicyclists wave me through, I ignore them. There is no way that they can truly judge the space that I need without running into an oncoming car or them. 

Same goes for when I'm in my car. I trust my own judgment. If it means that I am driving behind them for a bit, so be it. 

On my bike, I keep enough space between me and the curb and let them make their move. What if I wave them on, they hesitate, and then go after the golden moment has come and gone? Or if they pass a little too close because they are nervous? At worst, I'll stop and let them pass.  As we all know, folks in the cars don't make the best calls but better than me making it for them.