whether we should plan to attack castled king first or eliminate opposition's central pawns first....???

In Holland we do have a saying " to compare apples with pears " , when two absolute different cases are compared.                                                       Here we have even four items : attacking, eliminating, the king and the pawns.   The question "what first" is not relevant without more details.        So, please give more info !

That question is so vague as to be meaningless.

What you do is entirely based on the position at hand.

If you can checkmate your opponent then you should do so. It's foolish to not checkmate your opponent just because you can win a few central pawns.

If you can not checkmate your opponent, and your opponent can then open up the center while you're off storming the king, you may be in trouble. 

attack first. defend if needed. chess is more fun with open, attacking, tactical play. positional, defensive chess is boring. granted, i suck at chess though.