Having done this on a single-socket system, it was easier than I thought. But, no delidding. I watched a couple of videos, which at some point varied from the Mac Pro Technician Guide. I am very comfortable taking things apart and putting things back together, and have a sense for how tight is "tight." No need to buy a 3mm allen driver, because I already had 2 in my various toolboxes. Elapsed time was about 20 minutes.
All that said, be very familiar with how things come apart (lids and coolers), thoroughly clean up thermal paste and re-apply, tighten the 3mm screws according to the guide, and to spec. Make sure they are seated properly. You cannot be too careful.