Calculated risk. I've left 15mm of spacers on there, with the stem pointing down. If 15mm of spacers and a stem flip can't get the bars high enough then:
The frame's the wrong size
I'll give up cycling and just do stretching until my flexibility improves
I'll go for a second opinion
I'm sufficiently confident, however, that my existing bikes are sufficiently close to what I need that a bike fit will involve fine tuning rather than radical changes. And if I'm wrong then meh.
I assumed as much (: