You are right in thinking that on Cunard the grade of cabin you book dictates the dining room you eat in - Britannia Restaurant, Princess Grill and Queens Grill. In all three, you get allocated a fixed table for dinner, but in grill class, you also have the same table for breafast and lunch as well.
Our own preference is for the same table for all meals, but since the demise of the QE2, it's a more expensive option to go for on the remaining ships - it's better for getting to know your table companions and, of course, the stewards. Even if you are allocated a table for all meals, though, you're bound to eat elsewhere at times - on shore excursions, by the pool, in the Todd English Restaurant, so you don't get tired of your fellow diners. It's also less frenetic at mealtimes than queuing outside the restaurant waiting for the doors to open.
We've always found the level of service on Cunard excellent (except our first time on QM2 in 2004) and last year, superb, when we were upgraded to Queens Grill on QE2. It's very tempting to pay that extra for the next cruise!!
Paul & Andrew