Spreadsheet
So quick explanation: this is a spreadsheet that shows average number of hits on a roll of x dice, then shows the expected average result if you spend edge before rolling, after rolling to get rerolls, or after rolling to roll extra dice. It factors in the effects of exploding dice, and covers edge 1 through edge 8.
Your edge rating is the number on the top row, your dicepool before edge is the first column. The 0 edge column is the average roll without spending edge.
If a number is highlighted green you want to spend edge for extra dice (tab will tell you if this is before or after rolling). If the number is highlighted red then you should use edge to reroll, barring weighted dice.
This sheet does not factor in the odds of a chain exploding die (a die roll that rolls several 6's in a row), but whenever reroll and extra dice have statistically identical results I have recommended rolling more dice due to the outside chance of a secondary explosion.
As I understand it Edge in 6th edition works differently, hence I have set this as a 4E topic since that is the ruleset edge works like this in. Hope you find this helpful.