Me again, What I meant to ask is, can you use the bottom half of a dimensional shingle for the starter strip? Or could this cause a leaking problem? Thank you

There are 3 ways to start shingles:

1. A starter strip
2. A double layer of shingles (I don't like this method)
3. Shingles cut

Use Ice and water shield at edge and a drip edge. Some shingles aren't warrantied unless installed with Ice and Water shield.

I think the tabs of a dimensional shingle as a starter strip would probably cause unsightly bumps or bulges in the first course. You would have to start with half a tab to insure the joints between the tabs do not line up with the gaps between the tabs of the first course. Why would you want to do it. The savings in material would be exceeded by cost in labor. Save those tabs for the ridge cap.

Here is the general method w/o using a starter strip:

Applying Asphalt Shingles

I asked a roofer about the double-layer method that he used and he basically said it depends on how windy it gets and that method was OK to use for my locality.

Some install a starter strip by just reversing a normal shingle with the tap portion up. Some will cut off the tabs on a shingle to use that as a starter strip. A starter strip purchased is the best way to go because of reasons posted above.

YES - ICE and WATER Shield is very important.
It's now code in many states and there is a reason for it. I have seen plenty of reasons.

This is used along the eaves towards the edge of the roof. About 3 feet in width. Along the valley's and in other places. Keepitsmiplestupid is right on.