Here is some general info the members here can expand on:

Shy Trout: Most mountain stream trout require a small tippet because they are line shy. I have used leader tippet combinations as long as 12 feet with the last three feet being tiny tippet running down to tiny flies. The leader is almost always tapered. Consider that you may be using a 7x tippet which can be as light as 1.2lb test (depending on the manufacturer). The leader is going to provide the ability of the line to "turn over" the fly and the tioppet woill not do anything in that regard.

Bass and bluegill: This is a different world of fish who are not line shy. In most cases you will want to use the same diameter line from your fly line to your fly, perhaps adding a couple of feet of a slightly lighter line as a tippet, but this is not necessary.

Saltwater: Again we are not dealing with line shy fish. I almost never have tippet here unless I have a shock tippet to provide a more bite-proof line at the fly.