Like Pat, I found the postal employees who helped me with General Delivery stuff to be helpful, polite, and competent. We never had a problem, ever, despite using General Delivery quite a bit.
I also used general delivery for backpacking restock points often in "post offices" that were just a cubby in a general store or outpost. In those cases the people tending the "post office" were not actually postal employees, but they were generally helpful and in several cases were willing to retrieve my package for me even when the post office wasn't officially open.
The use of warmshowers hosts for me has always been one of those things that sounds attractive, but doesn't really pan out all that well in actual practice most of the time. My touring style is such that I avoid any rigid scheduling, I find a rigid schedule sucks much of the joy out of touring. I like being able to not decide where I will stop for the day even in the morning of that day. I might decide that I want to knock off at a 30 mile day or may feel like pushing for a 130 mile day. As a result I typically do not even know in what towns I will be actually staying in even the same day and definitely not exactly when I will pass through a town a few days down the road.
If using a post office you always have the option of changing the pickup location, by requesting that the package be sent further along on your route. With a warmshowers host I feel committed to be where I say i will be when I say i will be there. They most often quite reasonably expect notice of when and if you will be staying with them well ahead of when I typically like to make that kind of decision. I often don't decide for sure where I will be staying until I actually arrive there. I may roll into a town and decide to stop mid day or i may feel like riding late into the evening and knock out big miles. To me having to commit to a time and place a few days down the road just doesn't fit my preferred touring style at all.
I know that not everyone tours that way and some plan out every stop even before they hit the road. For them none of this is an issue, but it is worth considering whether it is for you or not.