Given that "a (twin) turboprop" can mean anything from a Twotter to a Turbolet to a C90 to a Be1900 to a Q400/ATR (not to mention a dozen others), I'd guess "optimal" covers a fairly wide range.
About a decade ago I saw (perhaps on these forums) an extremely rough rule of thumb for turboprops - related soley to most efficient climb/cruise/descent (but ignoring temps, winds, weight, direction of travel, etc.) - to the effect that 1000 feet per 10 miles of total flight distance would be close to the optimal cruise altitude to plan. E.G. - for a 150 mile leg, plan 14K to 16K feet.
Does that even come close to approximating reality anymore?