A one or two dollars in US or pesos for all your bags carried to the taxi or car is fine. If someone is picking you up (car service, hotel, or tour company), don't bother with the porters, someone will take your bags for you once you get outside.
The luggage carts at the carousel are free and there are plenty of them, so just grab your own bags and go. Even if you do this, the porters will still try to take your bags after you pass through customs/duty check...you don't have to use them...
I know a couple of guys from the states that come several times a year and love to tip, and tip based on US standards...they actually feel bad giving the bag boy at Playero "only" 100 pesos...I try to explain that's about a half day's salary for 2 minutes work, but they don't care...they'll double a taxi fare for a tip and tip the maid 1000 pesos when they leave after a weekend stay...so you can understand why taxi drivers, porters, moto guys, etc. think they can get away with asking for inflated "tips"...each one probably gets at least one "big spender" a day...
One interesting observation...when you you do tip here...whether it's 10 pesos or 500 pesos for the simplest service, you will almost NEVER get anything beyond a mumbled "thank you"...if that. This is one of the biggest little problems with DR tourism. In other countries, especially in the Caribbean, service workers know to smile and give a genuine thank you for gratuities...it makes a huge difference in most people's perspective of a tourist destination...even if it's only on the subconscious level.