A lot depends on your skiing style, personal preferences, and where you ski. I like a much burlier boot skiing lift-served than I do skiing backcountry. A stiffer boot is required for skiing big skis than is required for skiing mid-fat skis and skiing stiff skis vs softer skis and skiing faster, more agressively than skiing slower with more finesse and having to make lots of minor adjustment (as in the backcountry). Lift-served I like a progressive flex, a boot with alternate cuff lean angles, and one I can add a booster strap or take it off. I use an AT boot (Dynafit Zzeus) that is polyurethane with interchangeable toe blocks (pin blocks and alpine blocks) for lift served skiing. According to Lee Lau's list it has a flex rating of 85. Yet I skied 184 cm Kastle LX92s (92 waist), 191 2011 Volkl Mantras (98 waist), 188 Rossi S7 (118 waist?), and 190 cm Bibby Pros (118 waist) with no problem in all kinds of snow on piste and off piste. If the snow is good I use the lesser of lean angles, no booster strap, and a nice, mildly snug buckling. I am thinking of adding a new ski, say 106-112 w medium flex around 178 cm with tech bindings, for skiing deeper snow, principally in trees and glades where quickness and short to medium turns are needed and using my Dynafit Mercury boots--that will take about 10 lbs or so off my aging legs :-). The Mercury is stiffer than the Maestrales but I can ski it w/o the tongue and w/ or w/o a booster strap allowing for adjustment in stiffness; there is little in the way of progressive flex or flex of any kind but there are light.
In the backcountry I've used the TLT5 and TLT6 2-buckle boots without accessory tongues for soft skis from 76-98 mm waists. For stiffer skis up to 106 mm, I use the tongues and a booster strap. For my 184 Volkl Nanuqs (98 waist) and 182 Dynafit Stokes (106 waist), farily stiff bc skis designed to go fast, I prefered going up to a 3 buckle boot (Dynafit Mercury) w/ or w/o tongues and or booster strap depending on the snow.
Lighter, softer, looser boots are more comfortable for me and more easy to use when there are lots of terrain obstacles to avoid and some narrow spaces to ski. As the runs get longer, steeper, deeper, and the snow heavier I prefer increasing stiffness of boot. Removable tongues make climbing much much easier for me too.
Oh, btw, I weigh 230 lbs and was a pretty strong skier till I injured my lower right leg; I stiff found the Mercury to a little stiffer than I liked for skier lower angles and tighter places.