http://www.oople.com/rc/photos/cars/501x/hbwheelnuts/
I imagine the Tamiya ones are somewhat like the Hot Bodies ones above?
Anyone who tells you they will never come off isn't telling the whole story - but as long as the wheels are in good condition and mounted properly (and are GOOD wheels to begin with) then the serrated nuts are ace.
Strangely its only Tamiya that would have a problem with serrated nuts if you're using the old 501X wheels with the loose front hex. It could spin off one of these serrated nuts quicker than you could blink. New style front wheels are fine tho.