Do I really need to wait 4-6 wks before staining my new cedar fence? What if it rains?

The wood is that "green"?

If the wood was purchased from a wood supplier, it should be kiln dried and should be sealed as soon as possible.

Don't wait for it to rain.

Use a QUALITY sealer.

The better it is, the less often you will have to re-seal.

Most sealers will suck into the wood, so a good heavy coat should protect even the bottom (unsealed) edge, except, if this is a new fence, you should be able to seal it also.

It should not be in the dirt anywhere.

If it is, dig it out or it will destroy the wood, sealed or not.

If this fence is to keep animals inside, they are smart, don't let them see you pulling back the dirt from the bottom of the fence.

If it is pure cedar (untreated) , and it is dry then I don't see why you would need to wait. If the wood has been treated and still has wet treatment on it, then you have to wait until that dries/soaks into the wood. Rain will not bother that as much as you might think.

2nd part of your question: Yes, the bottoms are important. Cedar resists rot for about 7 years on average. But If it is treated and or stained then it will last longer.