Always.
The Wii can't sustain 60fps Super FX (most notably Doom, Star Fox 1 and especially 2) without hacks. That's why frameskip is on, with the setting previously hidden, as it works nicely without being too evident.
Two little known facts: 240p actually does help with the framerate in Snes9x by a fraction, around 2-3 fps tops. And the second most important, GX/TX inherited an unresolved timing bug from the PC code, so the Super FX games run "accelerated" at an incorrect pace, which RX does not suffer.
