i have always waited until that last day to see what we were going to do that day and if we needed them, added the extra day. use to be quick and easy and cheap.
now it depends where you purchased your tickets how easy and inexpensive it is.
meaning.........and i have not encountered this yet, but disney announced that they have changed the way they credit the value of your ticket if you purchased from a discounted like maple leaf or undercover tourist - like we do. now instead of crediting you with the value of a six day ticket and charging the difference between that and a 7 day ticket, they credit you the value of what the discounter paid to disney for that ticket, not even what you paid the discounter, but what the discounter paid.
so now instead of an extra $10 a day extra it is going to cost you a bunch- don't know what the discounters pay, but sure it's noticably less than you paid them, and what you paid them is noticably less than the value of the ticket. so you are really getting screwed..........
bottom line, if you bought from disney directly, wait until the last day and see if you want to add a day. if you bought from a discounter, consider doing something else that day