Suppose I have a 9/8 time signature and I need to a put a rest somewhere which is worth a dotted crotchet (quarter note). Do I write a crochet (quarter note) rest with a dot, or do I write a crochet (quarter note) rest and then a quaver (eighth note) rest separately?
It's the same thing. Dotted notes can be used in any time signature. The dotted crotchet signifies that the rest is for a crotchet and a quaver. If it fits the measure, it can be used, like in your example:
But you can also use two rests, a crotchet and a quaver:
If the grouping is 3-3-3 (quavers), I think the former case is more likely to appear.
It somewhat depends on the location of the rest. If the 9/8 is meant to be a threefold division of three beats, then the dotted quarter rest should only be used on beats 1, 4, or 7. If the division is (the rare) 4+4+1 eighth notes, then the dotted rest should only appear on beats 1 or 5. Similarly, a quarter rest probably ought not to start on beat 2 (unless the underlying division is 1+4+4 eighths but that is difficult to distinguish form 4+4+1.)
I personally never (rarely anyway) use dotted rests; I just find dotted rests harder to read.
It depends where the rest is.
- If the rest begins on the beat it should be a dotted crotchet rest.
- But if it's off the beat then it should be written separate quaver rests.
- However, if two consecutive quaver rests are at the start of a beat they can be combined.
These rules help show the start of each beat clearly.
As stated in Elaine Gould's Behind Bars, p. 163:
[R]ests at the beginning of a beat may be combined.
Rests that follow a beat should expose all three divisions[.]
There are only seven places in a measure of 9/8 that a rest worth a dotted crotchet can go, so it might be worth listing them:
(unless you consider notes smaller than quavers)