Having worked in QX ACC for a number of years, being an ocean controller as well as a planner, I can offer my take on this subject.
When NY calls QX with an estimate, if the aircraft is not on a track, the QX planner has to join or cross that traffic with the oceanic track traffic. A join or cross requires 15 minutes of procedural separation. Vertical is always the easiest answer, if a pilot indicates that he is able the requested level.
In my current unit, not QX, when we pass an estimate to NY, we are required to give them the highest level the aircraft is able to maintain prior to leaving radar coverage. This again, is to allow NY to accept the aircraft against the primarily east-west flow. It is easier to use vertical sometimes than to figure out the crossing point, determine if you have the required time standard, then monitor it constantly. In my experience, however, the requested FL is the one that is usually approved.
Unless things have changed in QX, the planner would have the actual altitude and requested altitude for the ocean in the database. They don't receive the max altitude. Fedex is a prime example of the problem. They regularly flight plan the same routing across the ocean, about 5 minutes apart, and both requesting FL330 in the daytime. So that means, either reroute or level change to FL290 for one of the flights.