Garb,

Not to make this too technical:

If you tie on some tippet material..and you can tie on as much as you want...how that effects your cast depends on your casting AND the type of presentation you are after...with dry flies it's probably more important to have a delicate turnover..for streamers, wets, or nymph fishing, not so much.

How 'often' you need to replace the tippet section depends entirely on how long it is and how often you change flies...you can see when it starts to get too short...the knot will get closer and closer to the end of the line...

Personally, I buy 7 1/2 foot 4X tapered leaders and tie on two to three feet of 4X or 5X tippet for most of my dry fly fishing..I can easily turn over three feet of tippet...and for really spooky fish, I've found that by concentrating I can delicately turn over up to six feet of 5X...I know some guys that use nine feet of tippet, and they can turn that over...I can't do that consistently yet (luckily, most of my dry fishing is for unpressured 'wild' fish...they tend to be a little less finicky).

For when I'm using nymphs, I'll use the same base 'leader', but go with 5X or 6X tippet, depending on the waters, but use a longer piece. This lets me get the whole section that will be under an indicator to a level piece of tippet....If I want the top fly to be four feet below the indicator, I'll tie on at least that much tippet....This isn't a 'delicate' presentation...hits the water like a handfull of gravel, but it's awfully effective for trout fishing.

There is no 'one correct way' to do all this....you'll find the ones you like will change as your experience grows.

Buddy