If someone gets paid without taxes being taken off in the paycheck (taxes will get taken off at the end of the year instead of each paycheck) should that person give Maaser money from each paycheck? Or should they wait until the end of the year when all the taxes will be taken and then give Maaser based on that amount?

The amount a person gives for maaser is calculated on his income after taxes, since taxes are an “expense” that is necessary to earn this income.

When you give the money to tzedaka is your decision–you can give it each month (and make the calculation how much you would be earning after taxes), or you could give it at the end of the year.