I always pronounce the letters. It's what you're supposed to do when abbreviating a title with the first letter of each word as in the case of Nintendo Entertainment System. I have heard people on YouTube and such say Ness, Sness, and even the more oddly sounding Snezz, which I guess they think sounds jazzier? But, does anyone pronounce I B M as ihbim (or something along those lines?). Or AT&T as ahtint? I've also never heard the PS4 pronounced Pzzfore. So, it doesn't really seem logical to me.