I wouldn't worry too much about it. As long its its firmly in the slot and screwed into the case, it shouldn't naturally bend enough to cause a problem.
My Gravis Ultrasound is quite long and does have some noticable sag but I'm not worried about it. A lot of VLB cards are quite long as well and sag also, despite the extra connectors holding them to the board. As long as they don't sag so much they touch neighbouring cards I don't think its a real problem.
PCB's should be able to flex a little without problems, although I'd definitely advise against using one to practice your Rolf Harris impressions.
Some modern video cards are quite long as well, and they are far more complex (more layers, smaller traces) than an old sound blaster, AND they have to support a heavy cooler as well, and I've never seen a bent one of those 😀
As HAL9000 says, if you're really concerned just get a desktop case or lay your tower on its side.
If you are squeamish, don't prod the beach rubble.