Concert etiquette

Odd things concerts.. It's rare (with the bands I see!) to have any amount of seating. Still at the smarter venues that are more like concert halls there is normally something in the upper tier. But often tickets are not sold as seated or standing - you get what you are given.

So, is it fair for me to just sit down on an empty seat or not (after all, ickets are all the same price)? How many seated ticket holders will be downstairs and bopping away whilst I yearn for seating (combination of bad back, age, and 5 gigs in one week). So I'll try tonight and see what happens!