Usually, as high as possible will generally be in your favor when it comes to fuel savings. The only exception is when the wind really starts to take a major chunk of your ground speed.
For comfort, try to plan 15 minutes of level cruise time at the minimum.
If the aircraft isn’t pressurized, the max is 12,500’ for cruise.
If there are icing conditions, keep your cruise altitude outside of it. If it’s a thick layer, stay below it. If it’s a few thousand feet, you could punch through it and cruise above it if your aircraft is certified for flight into known icing.
Europe has more specific IFR requirements for City-Pair level capping. See the Route Availability Document published by Eurocontrol for more information.
Sometimes if I feel like killing time, I’ll fly the whole route at the highest MEA enroute. Saves me the planning effort if someone already did obstacle clearance calculation for me.
Simbrief is a fantastic resource, it acts as an automatic dispatcher.