Nice to hear from some other 6.5 owners. It IS possible to get the grills to stay on. The main problem is the rubber grommets are initially too stiff and want to spit the screw head back out. You need to soften up the rubber somehow so it will grab the screw head and stay put. First, you have to install the plastic washers. This makes the new screws stick out far enough to grab the rubber grommet. Next, get a stiff piece of metal about 1/4" diameter (I used the blunt end of a 3" hex bit from a cordless drill), and loosen the hole of each grommet up by working the piece of metal in a circular motion. Be careful not to go so deep into the grommet such that you touch the grill cloth. Once you get all of the grommets well loosened, you can press the grill on and it will stay. Make sure to press it on evenly and keep pressing it until it stays in place. The lower grills do warp a bit. I've made sure the upper 4 screws are well fastened and live with the bottom sticking out a little.
I think the effort is worth it. I lived with the bare drivers staring me in the face for 1.5 years before putting the grills on. The speakers look a lot better with the grills on. They don't affect the sound much either, so I think it's a good compromise.
Incidentally, my room is 18'x19' with a peaked ceiling that is 10.5' high max. The room has several openings and a fireplace in one corner which makes for very few parallel surfaces. It is carpeted with drywall walls. The 6.5s are 5' from the front wall and spaced 8' apart on centers. I sit 12' away from each speaker. The ridge of the ceiling is parallel to a line extending from my chair to the center of the speakers. The speakers are not centered in the room (dictated by practical traffic pattern considerations). The center of the left one is about 4.5' from the wall. This produces a very nice wide and deep soundstage that extends past the speaker boundaries.