Please don't take this the wrong way, but I'd look for another mechanic if the one you have actually puts a transmission dipstick to his cheek. They can get hot enough to burn the skin quickly. I'm hoping this is a story he told you and not something you actually witnessed. Otherwise, how many really macho slash scars does he have on his face?
Seriously, the level check after running for 5 minutes is because the temp will be close to 90C and hot temps can vary, depending on the season, how hard you were driving, etc... I understand the logic of checking after 5 minutes of warmup, the image of somebody putting a dipstick to their face, no way. My 300TE tranny dipstick gets hot enough in summer that I hold the thing with a rag to keep from burning my fingers!