I think all my effects will normal a mono input but some effects will further Stereo-ize.
Mimeophon has some nice ping-pong style features and it will also put the delay in the opposite channel to the initial signal.
Clouds has a knob for stereo seperation.
ZVERB is only mono-in but Stereo out.
Veno Echo does all sorts of stereo fun from a Mono input.
Pico DSP is mono in and Stereo out, think that depends on the mode you have set.
Enjoy your spare HP, don't rush to fill every last space, this is not like filling sticker books. Resist the urge to 'complete' your rack, its never complete so just relax.