hmmm, well slap my ass and call me sally...
I know I've read what I wrote somewhere before, and it must have been a reasonably authoritative source for me to have repeated it like that but if you've been living this theory for years and never had a problem, maybe it's just an old wives' tale sort of thing.
Of course the devil's advocate in me would say with an open muzzle end there may be an even better chance of getting something lodged in the barrel than had the holster been closed.
All the holsters I use are kind of a hybrid, I guess. They cover the barrel and slide completely but still have a small opening at the bottom. Makes them super easy to clean if nothing else.
Alex, what you're asking for may not be possible with a full-size pistol. I'm 6' tall, 165 lbs with a slim frame and there is just simply no way for me to carry a full size pistol IWB comfortably. When I do carry my .45 I carry it in a galco concealable OWB, usually with a sweatshirt or something over it.
That's exactly the reason I bought my LCP. It disappears right into my front pocket and I barely even notice it's there. Only downside to pocket carry is it makes drawing from a seated position difficult, especially in a car.
If you're serious about carrying, I'd buy a slim and compact 9mm with something like a crossbreed IWB. You shouldn't have a problem carrying IWB with the right size gun.