What a load of fascinating answers.
I have been out a few times on some short very steep climbs over the last few days and tried both ways.
Suffice to say, I found that it all comes down to what feels right at the time.
Sometimes standing feels right when you're able to power up until you hit a break and then sit.
Other times I've felt like sitting and spinning and getting up with my lungs still in my body.
Like a lot of people have said, whatever works best. I personally find that standing is more fun and gets me there quicker so I shall continue doing that for as long as possible even if my body weight ratio calculation is screaming at me to sit!
Thank you all for these great answers.