Consider the following examples:
Is there a semantic difference between these two sentences? Or do they mean the same thing, with or without the comma before "too"? If not, what is the correct usage of the comma in context of "too"?
According to the Chicago Manual of Style (subscription-based, sorry), you need to use the comma before too "only when you want to emphasize an abrupt change of thought."
Here's an article that gives that quote and has other illuminating things to say on the subject: http://www.dailywritingtips.com/comma-before-too/
Edited to add example from the second link:
He didn’t know at first what hit him, but then, too, he hadn’t ever walked in a field strewn with garden rakes.
In most other cases, commas with this short adverb are unnecessary.
Use a comma before too if you aim to turn the sentence into a double entendre. The change in meaning is subtle and lends itself well to being snide. It's not so subtle this is difficult though. It's pretty straightforward.
Example:
It's a bottle. - description of an object
It's a bottle too. - bottles bottles more bottles
It's a bottle, too. - it's a pencil case and it's a bottle
Another example:
I like you. - thank you so much
I like you too. - mate, we're your fandom
I like you, too. - but I'm also very jealous of you
I like you, too. - but I'm married to your your sister.
I like you too. - we feel the same about one another
The classics (no need for commas - spoken English alert). Requires common sense. Potentially abusive:
It was a pleasure to meet you, too.
It's lovely to hear from you, too.
I love you, too.
I missed you, too.
(Be especially wary. Comma too = as well something else )
