Best time to bluff is when approaching the bubble and again before approaching the final table. During those times most people are playing very tight and only calling with big hands.
the best spots of bluffing are on dry flops like for example 10 7 2 rainbow and everybody checks, also when you have position and there are few players on the pot ( 1 or 2 even 3) and all of them checks or show weakness like betting one third of the pot or even less. also you can try to bluff more when you have a big chipstack near the bubble against middle stack players who play very tight and only want to be itm instead of winning the tournament. you should often try to bluff tight players on low flops because most of the times they wont hit anything and even if they hit a set or have a pair you can bet and fold if they re raise your bet.
I think bluffing in position is always preferred. Try to mix it up, between value bet bluffs and bigger bluffs. Nothing wrong with betting to see where you stand in the hand.
For me the factors for a good or bad spot steal are : Position,Stack(medium stack 10~20 is the best to steal ), situation of tournament, playing style of opponent