421C and Droopystop are right.
I don't want to sound over dramatic but IFR in IMC in mountainous terrain outside of an airway or published approach procedure seems a pretty good recipe for CFIT.
Crossing the Alps for example, I would choose to be in an airway in class A controlled airspace preferably above the weather but even if in IMC weather I would know I wasn't going to hit the granite.
I have TAWS and use it as a backup, I would definitely have it switched on while flying through the mountains at night even if in an airway, just in case the engine quit, but would be very unhappy picking my way through the passes in weather using GPS and TAWS.
SB
PS: IO540, I think IF approach means instrument approach.