As 411A addressed, higher is better? This is the oldest theory regarding the most economical altitude for any jet.
Why let theory lead you into a very high workload situation where it is your decision to limit, distribute the workload in a manner which seems reasonable and prudent as PIC, never mind any compromise because of fatigue and/or weather, thereby allowing some possibility of CRM back-up?
On one of our flight plans for a very short leg, it stated that the fuel burn at about 12,000' was the same fuel burn as a climb to FL200, followed by an immediate descent. The flight planning software apparently was created by Jeppesen for airline Dispatchers. Why work your butt off to go up then back down like a 105 mm. howitzer shell if it is three times the work, especially for the non-flying pilot in a plane with no automation except for altitude hold [
]? Navigating, figuring crossing restrctions, setting hydraulic pumps, manual cabin, ATIS etc; none of this is done automatically (remembering to somewhere check the remaining fuel) . This does not include distractions from the cabin or weather, nor a nagging cabin altitude that won't descend at idle with airfoil anti-ice on.