We've just returned from our Liberty 8 Day Eastern Cruise. Had a wonderful time and I can now answer my own question with complete certainty.
The only way to make the bar credit coupons work in your favor (economically speaking) is to purchase drinks that cost exactly 5.25. There aren't too many. However, the coupon mentions that it covers gratuity (which is usually automatically added at 15%). So, you save 15% every time you order a drink if it costs $5.25.
It doesn't work economically in other situations. For example, many drinks were $3.75. The coupon would cover the drink and 15% gratuity, and you would get no change back.
Another example, You order the drink of the day with a souvenir cup for $6.75, or $7 something (it's a blur), you would owe the ship the difference.
We tried to combine some coupons on several occasions to purchase a bottle of wine, but the bar server in the dinning room would not let us. Maybe they didn't know we could, or you just can't.
Either way, we had a great time and now you (and I know).
It seems to make sense to buy 4-8 coupons and then throughout the cruise, use them on the drinks that at at least 5.25. Use your sail & sign card for the rest.