Random thoughts on this topic.
91.135 only applies to Class A airspace.
http://www.faa.gov/airports_airtraf...s/atpubs/Atc/Chp7/atc0701.html#atc0701.html.1
7-1-1. CLASS A AIRSPACE RESTRICTIONS
Do not apply visual separation or issue VFR or "VFR-on-top" clearances in Class A airspace.
I found this on a random webpage (old info).
(b) When operating above 18,000 feet MSL to flight level 290 (inclusive) and --
(1) On a magnetic course of zero degrees through 179 degrees, any odd flight level +500 feet (such as 195, 215, or 235); or
(2) On a magnetic course of 180 degrees through 359 degrees, any even flight level +500 feet (such as 185, 205, or 225).
(c) When operating above flight level 290 and --
(1) On a magnetic course of zero degrees through 179 degrees, any flight level, at 4,000-foot intervals, beginning at and including flight level 300 (such as flight level 300, 340, or 380); or
(2) On a magnetic course of 180 degrees through 359 degrees, any flight level, at 4,000-foot intervals, beginning at and including flight level 320 (such as flight level 320, 360, or 400).
This is the current 91.159 (faa.gov) that appears to have been amended in Oct and Dec 2003.
(b) When operating above 18,000 feet MSL, maintain the altitude or flight level assigned by ATC.
What about above FL600?