Hrqls: I forgot to post my own thoughts .. i will do so now before i read the replies :)
if i am 2 points from the end i will double from the start as i want my opponent to accept the double, so that i only have to win 1 game and i dont want to forget to double :)
if i am 3 points from the end of the match then i will wait before i double .. i will most likely have to win 2 games to win the match, so if i arrive in a position where my opponent will decline the doubling, then i won 1 already. by doubling later in the game i hold the advantage of the cube .. if i get far behind then i will double anyway then i will double as well in the hope for a wonder :)
if i am more points behind .. then i am not sure :) .. i think its best to double early in even points from the end and later in odd points from the end ?