For fuel you are better off flying an equal distance higher rather than lower than OPT FL. That assumes all other things being equal, namely the winds. E.g. if you have to chose one FL above or below go above, it burns less.
However, as I said watch the winds. It's all about specific range at the end. The other consideration is the reduced margin between MMO and VLS (coffin corner), if it's going to be bumpy you should stay low and give yourself a better margin.
With regard MAX REC as you can see in the FCOM it gives you 0.3g margin. Technically you can fly higher, but it's not recommended unless you're sure it will be safe. And if you go above don't go over by more than a few hundred feet. Avoid it unless your sure of conditions and if the other alternatives are significantly worse off for your flight.
Also keep an eye on planned ISA and actual at the top of climb. If you take off from an airport with a high positive ISA then at top of climb if the ISA has decreased a recalculation by the FMS may result in REC MAX going up. So during climb expect this.
Lastly if you happen to fly longer distances over areas with significant changes in tropopause levels and therefore cruise temperatures, beware. An increase in temperature will obviously result in drops of both OPT and REC MAX.