is sawdust a good mulch for citrus in containers? does it cool the root from our very hot summer (reaches up to 50 c during the day)? does it help growth? does it supply with nutrients?
what kind of mulch is the best and when to apply?

I have some organic cedar mulch I purchased when I bought my trees but have yet to use it.

Is it necessary to put mulch on top of the soil for citrus? The weather does get quite hot here....in the 90's (sometimes 100), but I haven't noticed the plants suffering at all because of it. And the soil seems to hold water sufficiently, without the roots sitting with "wet feet" all the time.

Should I leave well enough alone and not bother with the mulch as long as my trees exhibit vigorous and healthy growth?

Mulch is not recommended for citrus--it encourages root rot. If used, you should keep it at least a foot from the trunk.