i do this sometimes especially if it is a new strain. this way i get a real feel for when to harvest. i also do this if i am low on bud or just cant wait to try something new. this will put stress on your plant but nothing really drastic. you will lose potential yield from whatever you cut off. i am not sure if this is such a great judge of the final product though especially when it comes to taste and smell. most strains deevelop their smells and tastes in the last few weeks of ripening, also most growers like to flush at least 10 days before harvest so if you take a sample before you flush, it will not be the same as the bud that gets flushed.
so simply: yes you can cut off a nug, yes it will affect yield, no it will not kill the plant or cause hermie etc. and no the this is not the most accurate measure of thee final product.