Comma placement in "I, too"

You can write both

I too have seen that movie.

and

I, too, have seen that movie.

You can even write

I have seen that movie too.

and

I have seen that movie, too.

You cannot, however, write

I too, have seen that movie.

When an adverb is in the middle of the sentence, you have to either put two commas around it, or you have to avoid commas altogether. When you put the commas in, you indicate that that the adverb is a disjunct. The decision whether it is a disjunct or not is sometimes up to you. You can make it an adjunct (the opposite of disjunct) when you want to emphasize the connection of the adverb to the words around it, or a disjunct when you want to emphasize that it applies to the whole clause.

In many forms of writing you should probably avoid an overly "flowery" style. Of all the ways to write the sentence in question, in my opinion the

I have seen that movie too.

version is the most casual and closest to what one would say in most day to day situations. You should use the other forms if the situations calls for it. A fine nuance may be important when you write a novel, for example.