Wombat,
I have had four instructors, mainly because because they keep ******ing off to Airlines and stuff. Anyway I reckon it has cost me five hours. The biggest problems were the use of three during my nav training. Everyone had his own ideas and I had to do a small re-learning exercise to 'accomodate' each of their preferences.
This has really wound me up. No matter what strategy I decided to adopt it just became tiresome trying to explain why I did what I did. Even if I tried to point out that I had been shown a different method during the briefing it became a ten minute discusion to resolve/explain/re-learn the issue. I have complained to the CFI about an apparent lack of school standard for the methods they teach.
If you can do "C" then do. If you are like me then "A" and "B" will be frustrating and costly for you.
Good luck!