Is it recommended to put color preserver on guide underwraps before wrapping on the guides, and dec wrap underwrap before wrapping the thread design?

If you're using underwraps you'd almost certainly want to put something on them before making the overwraps. This helps facilitate making the overwrap.

If you're using CP on the wraps, then go ahead and apply it to the underwrap. Otherwise, you may want to put a thin coat of epoxy on the underwrap, just enough to seal it before making the overwrap.

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is there any other purpose on putting finish on the underwraps other than it helps in wrapping the overwrap so it won't dig?

If you need tremendous durability, then putting down a coat first, rather than assuming a later coat, which may be required to penetrate 2 or 3 layers will do so, might be the better course of action.

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I find that applying a coat of CP to an underwrap is beneficial when cutting off the dec wrap during the tie off - the underwrap tends to stay in place and less hassle with the tie off.

You might want to try a new product called Formula D that has just com onto the market. We use it in our shop all the time look on Youtube under Victor Standridge and it explains how this products works. It is a quick drying with very to little no fumes. You also only need to use only 1 coat of Formula D instead of the 2 - 3 coats of the other products out there.