Advice from an amateur here but we have done it both ways:
1) 2005 - we went to Wiesn just before noon to see the parade etc. - this is a nice option because I enjoyed the whole festivities but we were meeting up with peeps in Fischer-Vroni and we only just got in at noon because we basically begged and said our friends were inside (which they were) but there were really no seats for us - that was just luck because all the other tents had huge queues - most stuff is already filled up by this point
2) Last year we went to Fischer-Vroni at about 9:30 a.m. and walked in without incident - but it picked up shortly after that and seats filled up - there were no seats left at noon when the beer was poured nor were they just letting people in. It can be fun with a good group of people - bring food to nibble, card games, etc. but you miss all the outside stuff and you are sitting for a long time.
So I think it depends on the people you are taking and what they like better, tradition or beer hehe.
In answer to some of the questions, no beer before noon, you can bring food in, most of the parade stuff is outside so you don't see much 'fest' by sitting in the tent (not sure about
Schottenhamel though).
The one thing they did in Fischer-Vroni was have a countdown to the beer which was kind of fun.