Is it good practice to wave following cars through when safe?
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.