I have to disagree some with this part.Learning Spanish when moving to this country is particularly important since the local impression is that every foreigner is rich and therefore welcome prey for exploitation and exorbitant charges for goods and services which have no fixed prices.
YES, the better your Spanish, the better off you are, BUT, that is only part of the answer.
You can get ripped off speaking Spanish just as easy, you are still a foreigner!
Which reminds me of my English speaking "Dominican" friend back in 1991. We were going to his home town of Rio San Juan from Sosua to visit and he said let him pay the fare else they will charge me more because I am an American. I let him, but as we were traveling I watched the other passengers to see what they were paying. For the return trip "my friend" asked me for the money to pay.
I said "No Thanks", that was the last time I trusted "my friend".
You also have to be street wise in this country (Very Important here).
Don't for a minute think that just knowing Spanish is going to always protect you.
Spanish or not, I am usually smart enough to know what I should pay.
I always translate the prices to U.S. dollars automatically on everything.
Just remember that "My friend", "Today Only" means they are dangling the hook in front of you.
Speaking Spanish is not what will save you from exorbitant charges.
Being street wise is what will save you.
The answer to the question is:
You should know at least SOME Spanish and be very street wise to live in this country.
Don