At the end of the day, the measurements you post here are just numbers on an internet forum. How you measure your dick isn't going to affect the actual size one jot. The important thing is to be consistent so you can tell when you've made real gains.

Having said that, of course it's better if everyone measures the same way, otherwise some guys will have inflated numbers that can make it seem like the average size on here is bigger than it is, and that can be disheartening for a newcomer or someone with smaller starting stats. Use a rigid ruler, hold your dick out straight if you have a curve and measure down the middle on the top. Simple.