Charge from USB on computer or wall socket? Is it OK if I keep charging my iPad and iPhone using the Mac USB ports? Or is it better to charge them with the supplied chargers?

I understand that charging slowly is better (though I'm not sure why), and I don't think it can get any slower than 1 day for the iPad using the USB port.
Charging more slowly won't make any difference, other than reducing the slight side effect of heat buildup on the iPad.

But it's perfectly OK to use lower powered ports such as those on your computer instead of the main powered chargers.

It's also perfectly OK to do it the other way around; you can charge your iPhone using the more powerful 20W iPad main adapter. It won't actually charge any faster than using the equivalent iPhone adapter, though. People will warn you that you'll damage your iPhone using this method, but that's simply a misunderstanding. 20W is the amount that the charger is capable of delivering, but if the phone doesn't ask for it, it doesn't get it. It's not all or nothing.