The real issue is really to have a fin that the flex/bend/twist character designed into the fin is matching the load the fin is being used under.
Of course, in the matrix of fin options, many things "work", so it comes down to summarising trends -or massive detail on individual fin / rider weight / technique / sailing mode / board / conditions level.
A few years ago, when FW first started out, there was very much a trend to stiffer light wind and softer hi wind fins, for maybe obvious reasons.. But as fin and foil development went further ahead, it was better to restrict (stiffer) the deflection under higher loads to try and keep the foil working more efficiently (and so better control, less spin out etc)- whilst in lighter conditions, too stiff a fin was found to not be responsive enough (under lighter loads) to reach or maintain best efficiency.
Today, the trend is towards less deflection in higher wind fins, and some medium controlled deflection in light to mid wind fins. So it's the opposite of what could have been said a few years back..
Of course, the real key is to have a fin that that is designed to allow for the right deflection at that given load point, deflects just right for the relevant load being applied, allowing the fin to be at it's best under the expected (tuned) working load. So the finer points again come back to finding the perfect combo, which in turn is why the whole "ideal" fin selection thing remains a bit of a personal (style/weight/technique) and conditions relative equation. Some would say black art, with simple answers for generalisations, but very difficult ones for specifics.
A great way to gain some more experience is to borrow some (suitable size) fins from fellow surfers at your location, take a few combos for spin and see if you can determine a better performance (or not).
Pretty quickly you should start to get the feel, and from there you begin the life long quest for that holy windsurfing grail - the "perfect fin".
(always sought... rarely found)
Cheers ~ Ian