You should always tell them - otherwise they might not even know. And you can very easily be cool about it, no one said you have to call them with an attitude and say "fuck your shit wouldn't get a kite high!"
People who prefer to avoid even the slightest confrontation are just being cowards and are happy to live life just taking whatever comes their way. I say people should always at least TRY to get what they want - if you politely let your dealer know that his stuff isn't as good as it used to be, and he throws a shit fit, then that's HIS problem and isn't someone you should be doing business with anyway.
However, the BEST part about telling your dealer is this: I once let him know that the last few batches of tar weren't as good as it was just a few weeks ago... And he admitted that a few others said the same thing and he was actually kind of embarrassed about it. A week later, he told me it was back to the quality stuff and to make it up to me he gave me a really good discount on my next batch!
You'll probably find that a lot of dealers will want to make it up to you in a similar fashion - after all, they don't want to loose you as a customer!
So if you start getting bad shit, be polite, let them know and await your reward
PS: by the way, from a psychological standpoint, always try to deliver bad news with positive language - instead of saying "lately your drugs have been shit," say "your stuff used to be so good, has something changed?" Putting a positive spin on things will always get you a better reaction.