You need at least 1.5" (some say 2") of air space between the insulation and the roof decking so that the roof decking can always dry quickly, and wintertime moist air leakage that may migrate up from the conditioned space during winter is always diluted by vented attic air (that tracks the outdoor dew point) before it can condense on the roof decking. This is true in any vented attic, whether gable-vented or soffit/ridge vented. There are commercially made chutes that get tacked to the roof decking specifically to guarantee a minimum clearance for that purpose.
Blown insulation needs to be the same depth every where to get the full benefit, and this is an issue when going for higher-R when there isn't sufficient clearance between the top of the studwall and roof decking. Blown insulation tends to run between R3/inch and R3.5/inch, and if foot (R36-R42) needs to taper down to 6" (R18-R21) at the top of the stud plate it's a significant performance hit. This is because every square foot at the edge has 2x the heat loss/gain as in the deeper center, drastically reducing the average performance.
To mitigate that you can stack rigid iso insulation (R6/inch, or R36 @ 6" thickness) at the edges and use rigid iso as the clearance chute to bring the R-value at the tapered edges more in line with the rest of the attic.