I tend to do both surfaces because better safe than sorry. Also, by doing both surfaces I can apply thinner amounts and I find it’s less messy in terms of application.

I mostly use hot hide glue. If I need more time for a complicated glue-up then I just add salt which works phenomenally well and takes all the (brain) stress out. As a woodworking when I have a spare moment kind of guy I cook up my glue in a large batch and then pour it into silicone ice cube trays and freeze it. Then I pop those into a container and store the cubes in the freezer. When I know I will glue something up I just have to grab the appropriate amount and put them in the pot (glass spice jar in a baby bottle warmer) 15 minutes before I need the glue. Whatever glue is left over stays in the jar and goes in the fridge. If I don’t know that I’ll need glue in the next couple weeks then I just pour it into an ice cube tray and re-freeze it and put it back with the others in the freezer. I used to keep a bottle of LHG around for convenience, but when my last bottle got old I just didn’t buy it anymore as I just was rarely using it. The ice cube thing has been convenient enough for me.

I think they didn’t figure out that you could extend the open time of hide glue with urea or salt until the middle of the 19th century. Before that if you had more glue to apply than was humanly possible in the short amount of time available you had to heat up the wood parts and/or the entire shop. To me that sounds like a gigantic pain the butt (and expensive trying to heat the whole place up).