+1

On the trainer I stand, even if it's just for a pedal stroke or two, maybe every 5-30 min. I can and have done 5-6 hour trainer rides, although my normal trainer rides are 1-2 hours.

Think about when you stand or unweight your saddle, like if you put your foot down at a stop sign or light, when you stand to get going from a stop or a corner where you coasted a bit, a short steep rise where you don't bother shifting, etc. Most riders stand up much more than they realize.

When you coast on the bike you may coast in a non-load bearing fashion, bike tilted to the side or something. On the trainer when riders coast they usually stay planted on the saddle.

Finally when you go hard you naturally unweight the saddle. If you're putting a lot of downforce onto the pedals you will be supporting your bodyweight just a bit. If your trainer rides are steady and not too hard then you're not going to be making those efforts and therefore you won't be unweighting your saddle a touch.

hope this helps