I do about 4" hem - but doubled, the folded over bit goes right to the bottom. That gives a bit of extra weight.
If I'm using tape on the top the fold over is about an inch, and the tape is folded in about an inch at each ens. There's no point in using more, but when buying it is necessary to add a small amount especially if it's the type which pulls up to form triple pleats or something, like allowing for a pattern repeat.
The last pair of curtains I made have goblet pleats done without tape - and they passed inspection by DD's beady eye!
(she makes 'em professionally)
I don't think it really matters whether you do the top first or the bottom, although if they are floor length and you want to be sure of the exact length you might want to do the top first, hang them and then check for length. Likewise with shirt collar and sleeves, I was taught that it doesn't matter which is done first as neither interferes with the making of the other.