almost always, yes, even when the source is reliable.
however, recently we unexpectedly needed some speed, had no tester w/ us, and got given a 'methbomb' by an aquaintance (who's not a dealer) that we did not expect to lie to us. turned out to be ketamine - not that i mind that too much, but not what i was expecting.
so lesson learned.. always take the tester out even if you dont think you'll need it, and always test something.. or dont eat it.