I sometimes rotate pins slightly, just with my fingers, to help start them straight, but I'm not sure it helps. The main thing is to keep holding it up and looking at it to see if it looks straight, until you have it moving.
The piston is made from aluminium and is therefore not all that easily corroded, but the gudgeon pin is steel and corrodes very easily. That means you can't heat the piston with hot water. An ordinary oven at 150 Fahrenheit is good, but wear gloves so you don't burn your hand (150F shouldn't raise a blister, but it is too hot for comfort). Aluminium expands a lot with temperature, so warming up the piston will help considerably. Do not heat the pin, of course, you don't want that to expand.
So far as smacking the pin with the side or heel of your hand is concerned, you won't need to do it if you warm up the piston - the ladies on the production line didn't warm them at all. Imagine the kind of smack a small genteel lady could carry out every few seconds all day without making her hand sore. It is far from being a karate chop. The problem you had was a combination of not having the pin exactly straight, and having raised a burr with your efforts to remove the circlips.
