I generally prefer it cooler as the ideally sleeping temperature is cooler than many like, and it's always easier to add a blanket than to take off all clothing on an airplane.

My last two longhaul flights this month were pleasantly cool. During the summer I find it generally worse but usually have a nice chat with the cabin crew that it's too warm.

Often it's substantially cooler in the cabin galley area due to the lack of laptops and IFE. And as much as I love 64K and 1A on the 747, they tend to be fairly warm.