Personally if my bf was best man at a wedding where I didn't know anyone very well, I wouldn't want to go to the church or day 'do'. He will be busy with best man duties and I would be sat like a lemon, not knowing anyone! I would be perfectly happy meeting him later for the evening reception.
In fact, thinking about it I have done it, about 15 years ago. My bf of the time was usher at his brothers wedding and his sister was one of the bridesmaids. I was sat at the front of the church with his parents (first time I had met them) and as the bride came down the aisle followed by his sister as bridesmaid, his sister turned round and hissed to her mum 'Who is THAT?'...
Before the wedding, we had gone round to his parents house where he promptly disappeared to the pub with the men for a pre-wedding drink and left me with his mum.
And at the reception he went on the p!ss with his mates and left me with his mum and dad again and his sister just kep glaring and never spoke to me once!
Just to add, he didn't last long after that...