It depends on the horse for me. On my share we can go any position, if we are on nanny duty we can go at the front and keep it at a nice controlled pace, if we are out with the faster ponies/TB's we go at the back so as not to hold them up (though if we are on a track that doesn't have a 'stop' at the end I make very sure everyone I'm with has brakes). Going side by side with the TB we hack out with is fun, my boy thinks he might actually beat the TB (not realising the TB is being held back) so really goes for it!
The pony I ride get's VERY excited at canter spots and bounces and leaps about like crazy. If thinks he is being left behind it gets a million times worse and he usually ends up circling backwards at a canter, so I request to be at the front for the start. If he starts side by side his jumping about includes 'taking out' the competition (he side-swipes them!) so I usually try to avoid that!
With youngsters or new horses I tend to prefer to be at the back so that I have another horses bum for a brake!
Although I have agreed to go back to my RS for a hack with my friends I used to ride with and I'm pretty nervous about it. I'm now so used to being in control of the situation I dont think I will enjoy being part of a ride where I'm not. I could request to go lead which will give me more 'control'.