Like the other posters said, if there is a charge pump, stay under 10V. The Valhalla is an exception, it has a series regulator that protects the charge pump.
Some pedals will not bias correctly at voltages other than 9V. They might work, but will sound different. Examples are Buzzaround, BossTone, Tone Benders, Fuzz Faces... JFET-based pedals with trimpots will need to be retuned if you change the power supply voltage. Some, like the DLS mk III (Covert) were designed to work at 9 or 18V.
Read the manual for the original pedal, that should give you an idea if higher power supply voltages are recommended or dangerous.
If you're not sure it's safe, stay under 10V.