I think MasterAtWork answered this already...Quote from stoic:
Yes Market prices are meaningful, and my positions are marked-to-market every busines day by the broker.
So.....that relates to my question......how...?
If you're using mkt prices and intend to use your greeks to make trading decisions and explain the pnl on your marked-to-mkt positions, you have to use market implied vols. There can be no alternative...
If, on the other hand, you're a real money, accrual accounting type of investor, IV for calculating your greeks would be of very little use to you. In this case you can use whatever your heart desires as your volatility input.