There are a number of factors that may affect the selection of a line within a rods grain window:
Type of fly line - Typically skagit and traditional spey lines will be towards the higher end of the grain window whilst shooting heads will be middle to lower end of the grain window.
Type of spey cast - Touch and go casts like a single spey would generally be better with a line lower in the grain window with water bourne casts like a snap t or double spey better with lines higher in the grain window.
Casting style - Do you cast more with the top hand, with both hands evenly, or more with the bottom hand. Do you use a short stroke or a longer stroke length. Do you like to hit the rod or do you have a more relaxed casting style.
Casting skill - Both fast action and full flex or through action rods require more casting skill. A fast action rod requires better timing but is more forgiving of poor power application. A through action rod is more forgiving in terms of timing but requires good power application. A mid to tip rod is probably the best all round rod especially for a less experienced caster.
Personal preference - Some people like to cast off the tip off the rod and would typically use a lighter line than people who like a deep load.
Do not confuse a rods recovery speed with its action as they are two different things.
Personally I like mid to tip action rods, that bend nicely into the blank and have a fast recovery, as they are more forgiving in real fishing situations when I am deep wading or have restricted casting room. Full action rods are great with plenty of casting room but I struggle with them when under pressure. I also like to hit the rod a bit and do not have what might be know as a relaxed casting stroke. I generally fish with shooting heads in the lower to middle end of the grain window and skagit/traditional spey lines in the middle to upper end of the grain window.
All rod actions will cast all types of lines anywhere within the grain window, and probably some outside the grain window, depending on caster skill and preference, but some will be better suited to certain lines types and/or casting skill/style.
Hope this helps.