I prefer to let the transmission do the downshift. I believe this is better for the transmission with less wear and tear for the driveline. If you just pull the lever sure it will just shift immediately and on my truck it can be a bit harsh at higher rpm and near full throttle. If I let the computer decide then the shift is much smoother.
However once the downshift is made I do tend to pull the lever and hold the gear to the top to avoid unneeded hunting up and down causing more wear.
For a fairly long and steady hill I will often set the cruise to a certain RPM (3500 maybe) and let it run to the top with transmission held.
Ok mine is an old 4 speed. For those with 6, 8, 10 speeds it might be just as easy to set the cruise and let it do its thing in automatic. Just let it run unless it starts to hunt then pull the lever at the next downshift. OK to reduce the CC speed a bit on a longer sustained climb. Don't make a race out of it and you will be fine.