A lot of people still believe this but it just ain’t so. There was some validity to it long ago, before refrigeration, when oysters headed to market were more likely to spoil because of the warm weather. And back then, oysters were wild-harvested and there was a greater chance of them being contaminated with red tide or or bacteria, which flourish in warm water. With today’s regulated farming techniques, oysters are monitored for contamination. And, of course, they can be kept nice and cold from ocean to table.
Safety aside, however, some oyster aficionados stay away from oysters from May to August because of the taste–they claim that cold month oysters are tastier and plumper. Be that as it may, don’t let the time of year deter you if you are jonesing for oysters.
I used to do this for years, also with mussels. Turns out it is usually a waste of perfectly good shellfish. An unopened clam may be bad, but in my experience they are usually fine. What I do is set the unopened ones aside and deal with them separately. In almost all cases they are very easy to open with a blunt knife and the little feller inside is just fine. When you do find a bad one, out it goes. If you find more than a very occasional bad one, you might want to buy your clams elsewhere.
As an aside, fresh clams should always be stored in the fridge in a ventilated container, never a sealed plastic bag. They are alive and use some oxygen – not much, but a sealed bag will suffocate them and then you will have a bunch of bad clams!