Well, in the case you state, KJ, it shouldn't be a whole rest at any rate. In the modern notation a whole rest means the whole bar of rest, however long the bar is, so in 5/4, a whole rest is five beats, not four. (I know, that one weirds me out too…) (: So if you wanted a full measure of rest in 3/4, you could write a whole rest, or a dotted half. A whole rest doesn't have a specific metrical value. It's just the length of a measure.
Or am I missing something?