I’m told that Jack Lyon of Editorium discerns a semantic difference between, for instance, “I like ice cream too” and “I like ice cream, too.” Although I don’t know that I would set it down as a hard and fast rule, I do find that it works for me as for him: for me, the comma in this case does reflect a difference in intonation and sense. Consider these two sentences:
I just met this great guy, really handsome, good with kids, got along with my son. He like horses too.
I just met this great guy, really handsome, good with kids, got along with my son. He likes horses, too.
Assuming that the “he” at the end in both cases refers to the guy, not the son, do you discern a difference? I do. For me, the first one means that he, as well as the son, likes horses. (If we left out the bit about the son, we’d assume that the speaker likes horses and the guy does too.) The second one means that liking horses comes in addition to all his other wonderful qualities, without implying that the son likes horses (though since the speaker thinks it a redeeming quality, we can assume that the speaker likes horses).
So: no comma means, to me, that some other person shares the same characteristic, whereas comma means that the characteristic adds to a list of other similarly valued characteristics.
It works with non-humans:
My new car looks great beside my other car, same colour, and it gets good gas mileage too.
My new car looks great beside my other car, same colour, and it gets good gas mileage, too.
The first can be rephrased as “it too gets good gas mileage” and the second can be rephrased as “as well, it gets good gas mileage.” The comma makes the “too” apply to the whole verb phrase, whereas the lack of a comma makes it apply to the subject.
In other words, Jack Lyon sees it that way and I see it that way too.
Addendum: This is meant more as observation than as prescription. Not everyone perceives the difference as Jack Lyon and I do, and so you can’t rely on the effectiveness of this little distinction, much less enforce it.