I have to admit that I sometimes have a little too much gear. But when it comes to masts it is good to collect a few. The basics are to use the mast the sail maker recommends.
But here is a big but. No sail is developed for all conditions, sailor’s weights, riding style or intend of use. Most sails are tested for slalom, some brands have only heavy sailors, some have lighter. Many are doing the test-work at Kanaha on Maui. I guess some avoid the 9,5m2 days, and they never have flat water.
In the eighties I was sailing raceboard with sails from Tushingham, their racing sail was terrible and the only mast that made the sail work OK was Serfiac Pro Lite with the top cut off 30 cm. Took a while to find that out and hard and expensive to get the masts from France.
The last years when I was sailing Naish I found out that the sails worked better with softer masts, especially in the top of the sail. That may sounds weird since I am a heavy guy, and when it was possible I used RDM even on the big sails. Now when I am on The Loft Sails, where all sails comes with both RDM and SDM-cambers, I thought RDM would be perfect for me, but the SDM-masts are faster on all kind of sailing. (So far).
For most sailors the recommended masts will do the work. Especially if you are into slalom and not are committed to find the extra knot on the top.
The general rule is light riders - soft masts, and heavy – stiffer. But remember it could be the opposite. When it comes to tuning the sail, it is good the have some ideas and basic knowledge in sail design. You also have to know what you are looking for. Low-end acceleration, middle-end acceleration, top-speed, control etc. I.e. the right mast can give you the turbo-kick you want when the gust comes!
Don’t be afraid to test softer or stiffer masts in your sail. Sometimes you find a golden nugget. That nugget could help you mentally when you race. And if you think that you are fast, you will be fast too!