The only way you can begin to know is if you put it at the minimum and that's not reliable because you can't replay the same ties to check. It's very difficult to know what really makes a difference: would you have won so many with it at the minimum....perhaps. Even someone who sees a change in their fortunes when upping the bonus might have achieved that anyway without the bonus.

I've got bonus at the max and have achieved little - but it doesn't prove anything because I might have achieved less without it.

I work on the principle that if it might give me an advantage then I do it.