It all depends on the rider. For a cyclist like myself who weighs 213 lbs, I prefer to sit on those long uphills but some lighter riders like to stand; it affords them that little extra push. It takes a heck of a lot of energy to power your way up big hills standing and it it hard on older knees...so I shy away from it. I sometime stand and sprint for short climbs but certainly nothing over 500 meters.

I like standing. It is more tiring, but for me to gear down and spin my way up the hill, I feel like I'm riding in soft sand, like I'm not getting anywhere. On a REALLY long hill, I may alternate, but for the most part I'll do the entire hill standing on hills up to a mile in length. Now If we're talking about the gentle grade changes, the small rolling type hills that require only 1 or 2 gear changes, I'll just stay seated. It's the ones where I have to drop into the small chainring that get me out of the saddle. Also the little abrupt ones you sometimes get, you know, the ones that require a dozen crank strokes to get over, those I just stay in the same gear, stand up briefly and power over. They're small enough that they don't put me in an anaerobic zone and the recovery from the extra effort is usually pretty brief.

Let's go riding!

Max

I like to sit. I have a lot of power and can really go on long gradual climbs. Really steep climbs I have a tendency to fall off the back. I have been working more on standing climbing for the really steep stuff.
It depends on the hill and on the rider. When I biked across Canada we rode all the mountain passes in British Columbia. For these 20 miles climbs the best thing to do was stay seated and then stand for the shorter steep sections.
There is a local bike race that goes up a steep 750 yard climb. For this hill you have to stand.
In training do both and get good at doing both. In the race you need to analyze and figure out what is best.

We have some bridges locally that are about 1/4m in length to get up to the top. I used to stand but now I sit and push/pull with each leg and go faster than if I stand.

Don