No rule of thumb other than quality and what sounds good when you try it. Many designs operate on the assumption that they will be used for electric guitar so any frequency above 4KHz is largely ignored when testing. Some of these will sound dull on acoustic guitars, others will have the high end, but it may be harsh/brittle. Maybe Matt can tell you more about the hows and whys.
Largely this has to do with impedance, impedance changes tone, for example many microphone preamp have selectable input impedance, changing this allows you to create a different colour/character, outputs from ACTIVE pick-ups are generally alot hotter than passive pick-ups, lending themselves to different tonal qualities, normally a high/brighter sound. Some delay/reverb effects purposefully have a filter cap to limit the audio bandwidth, this is give the effect a more 'vintage' feel, this is done on MANY digital delays that are sold as analog delays, they add a filter to carve off the top end to make them sound more like the old analog bucket brigade chips... cool marketing feature ?
RE efffects and acoustics, sky is the limited, Wheatus used a Martin in a Marshall with a 4x12 to create the guitar tones on their hit song 'Teenage Dirtbag'... weird, but unique... I'm more of a puriest and tend to use effects sparingly with acoustics, a bit of modulation, univibe/trem, not a huge chorus fan on acoustics, a dash of verb and I have a loop pedal, I got Guy into using his with his drones, still use my FIRST generator RC20, with no 'undo' function... we have come along way!
Cheers
Matt
