I have never given anything but pain meds after a spay. I usually give them for about three days. These pups do get up and run around, but I still think they are hurting even though they often don't show it. It is their nature to hide pain.
I see no reason not to use a sling. I don't believe you can spoil them!
Spoiling is neglecting them...the word spoil to me is a negative one. Loving a pup is making him/her happy...not ruining/spoiling!