In my opinion (famous last words like "here hold my beer and watch this"), it does not matter THAT much how tightly you hold it for best accuracy, it matters how consistent you are at doing it every darn time. Do things the same way, every time and you will start shooting better.
Tip #2 is don't fake yourself out and think you are better then you really are. Put 50 shots in one target and you will really start testing your consistency. I see a lot of folks show a one mag group and I have to laugh. Its easy to shoot 5 or 10 shots in a row consistently, but try and put 50 one the same paper with out screwing up and then you will see your real accuracy come out. Remember consistency!
Here is 50 shots at 15 yards offhand:
Not the best, but a reasonable target for just blasting away!