Another area of concern when flying so close to max altitude is if you get a TCAS RA "climb" warning.. If you subsequently went straight into coffin corner you may then have the added drama of a stall warning, which overrides the TCAS climb warning. Not a comfortable situation. I was always taught to leave at least 600' spare.
I realise many of these routes are one-way and that you shouldn't get such a warning, but with fuel such an issue I note many aircraft at non-standard levels these days in an attempt to fly at or near optimum.