Bit of both for me really.
I will ask for some outline at various points around the hack, and how long I ask them to hold it for depends on how well-established the horse in question is. I.e. Skippy is just coming back into full work after just under a year on and off work. Therefore he has no real muscle or topline to speak of as it all wasted away whilst he was off work, but he has been schooled to carry himself in an outline by me before he went lame, and this time in 2008 and late 2007 he was carrying himself in a correct outline for long periods of time, so he does know how to do it. So I'll ask him for 10 mins or so, then a break, then another 10 mins, then a break etc.
Soda on the other hand has been in regular work and working correctly for the past year and a bit (before that he was so green and unbalanced he couldn't manage, bless him)! so he has the correct muscles and can hold a correct outline for a longer period of time, so I ask him for longer, he will quite comfortably go round the long block (about 45 mins) in an outline (and I don't mean sawing on the reins, just a light consistent contact which he works into) I don't always ask for it, but more often than not he offers it to me out hacking, he has quite good natural self-carriage anyway, or seems to to me and my RI thinks so, so he doesn't seem to find it hard at all.
Both my horses are th types that tell people if they're uncomfortable anyway.
I do however always give them a long rein for the last 5 mins or so before we turn up the drive to stretch out ec. But then I will ask for some free walk on a long rein and a proper stretch before I allow them to dawdle along whichever way they want.
I don't see why not. We have good facilities at the yard and are very forunate in that respect, but that's no real reason (to me, not having a dig at anyone just my personal opinion) not to ask for a bit of correct work. For Skippy it is more beneficial to him to work in an outline as he has back problems anyway from a bad injury before I bought him (resulting in the chiropractor having to come out every 6 weeks now) so Skippy needs to be asked to work properly (according to the chiro anyway who has advised me that in the long-run Skips will benefit from working correctly). Soda has a natural self-carriage anyway (or seems to) and tends to work into my rein contact. He also has the tendency to take advantage when I have loose reins and fling his head around (back teeth and saddle all checked) and when I pick up a contact he immediately comes back to me and is happy as larry.
It's all personal opinion really. Personally I'd rather my horses worked correctly at most times, I do some schooling out hacking, they then work properly in most places, I feel it makes them concentrate on me more and I do a bit of leg yeild and shoulder-in on quiet roads and we practice walk to canter transitions and medium trot out in fields as well. Equally, I let them have a break and a dawdle at times on a loose rein, but they know when I then pick them up again, it's work time until I loosen the reins.