Comma before "too"

Do you really have to add a comma before the word too at the end of the sentence? And if too is in the middle of the sentence - do you have to put the comma before and after it?

Technically, no. It's really up to you. If you want to emphasize your thought then adding the comma(s) creates a pause. Take, for instance, this sentence.

The Mavericks suck. The Cowboys suck, too.

Here I have added emphasis to how much the Cowboys can go to hell. But I can also state how much Dallas in general sucks by not adding a comma.

The Mavericks suck. The Cowboys suck too.

The same can be said for the commas in the middle of a sentence: put them there if you want to emphasize, don't otherwise. These rules can also be applied to also, in addition, however, and therefore.