Just got back from Majestic Elegance for 5 days, and I would recommend bringing $75-100 in 1's/5's for tipping for that time. It starts at the airport, people want to take your bags for you and expect a tip. Shuttle drivers to and from the airport. Bell hops at the resorts who bring your bags to your rooms. While the bartenders and waitstaff certainly don't expect it, the ones that are friendly and great clearly really appreciate it.
We did not tip every drink or meal, but every couple drinks we would leave $1 or 2. If you plan on spending a long time at a certain bar, tipping well early is always a good idea.
If you go on an excursion, the driver to/from expects a tip, as well as the people who lead the excursion (and they deserve it).
As said above, it is not required to tip. But, when you realize how much they work and how little they make, you want to tip a little more.
Remember - $1 = 44 Pesos so 1 or 2 dollar tips are appreciated!