As zippyfusenet points out, the best altutude to attack at depends on the kind of opposition you are facing. The Japanese had much less effective light flak than the Germans. The US medium-bomber switch from low to high in NWE, and from high to low in SWPA, can be traced through the first few of the following snippets, which I have just collected from a swift flonk around the interwebs:
https://media.defense.gov/2017/Dec/29/2001861960/-1/-1/0/T_GANN_FIFTH_AIR_FORCE.PDF traces the development of medium bomber tactics on the South-West Pacific Area from ineffectual medium-level boming to devastating low-level skip-bombing and strafing with para-frags. Specific altitudes mentioned include:
Jul 42 over Gona, B-25s and B-26s at 25,000 feet.
Sep 42 over Buna, B-26s at 5,000 ft, A-20s at 70 feet.
Dec 42 report on anti-shipping attack recommends 150 to 300 feet.
Mar 43 Bismarck Sea against shipping, B-25s at 3,000 to 7,000 feet, and 300 feet and “masthead height”.
Aug 43 over Dagua, B-25s at 30 to 50 feet, over Boram, 100 feet.
“May 17, 1943. Eleven crews flying Martin B-26s take off from England for a low level bombing mission in Holland. One turns back because of mechanical difficulty. The remaining 10 aircraft are all shot down. This mission results in a change in tactics, as from that point on, Marauder crews bomb from medium altitudes of 10,000 to 15,000 feet where they will suffer only light losses to anti-aircraft fire.”
https://legionmagazine.com/en/2013/04/mitchell-men-medium-bombers-at-war-air-force-part-56/ has RCAF Mitchells operating usually at 10,000ft and below, but mentions one that got hit at 15,000ft over Venlo.
http://ww2today.com/tag/bombers mentions RAF Mosquitoes on the Oslo raid of Dec 1944 travelling to the target at 28,000 feet, descending to 3,000 feet in the target area, and attacking at 1,000 feet.
“Battle of Britain 1940: The Luftwaffe’s ‘Eagle Attack’”, by Douglas C. Dildy, says that Luftwaffe Do-17s and He-111s operated typically 13,000ft (4000m), from which height, if his accuracy figures are to be believed, they couldn’t hit a cow on the arse with a banjo.
https://www.raf.mod.uk/what-we-do/centre-for-air-and-space-power-studies/documents1/air-power-review-vol-18-no-2-battle-of-britain-75th-anniversary-special-edition/ includes a copy of Keith Park’s No 11 Group Instructions to Controllers No 10, which says:
“The Hurricanes, because of their inferior performance, should normally be put in against the enemy bombers, which are rarely above 16,000 feet by day.”
A D Harvey, “Army Air Force and Navy Air Force: Japanese Aviation and the Opening Phase of the War in the Far East”, War in History
Vol. 6, No. 2 (April 1999), pp. 174-204 (31 pages) mentions:
“25 000 feet, the altitude at which Japanese bombers commonly operated”
The 1948 London Gazette supplement on the loss of Repulse and Prince of Wales reports that Japanese level bombers were “remarkably accurate” bombing from 10,000 feet.
All the best,