With gmail, you don't need it - foo+bar@gmail.com will end up as foo@gmail.com and you can filter by To: header.
Dick move, I know. Tell marketing that though.
I personally use gmail through a vanity domain and have a catch all rule, so I end up signing up with a fake email account for every domain (hn@mydomain.com) and then the catch all forwards it to my real account (me@mydomain.com).
At which point you should wind up in the "how widely can I advertise that you're a spammer and all your outbound email should all be routed straight to /dev/null for sending mail to an email address you were never given" filter.
Which works, until the Gmail users who bother using + addresses with filters start giving all legitimate senders + addresses and sending everything thst doesn't have one and doesn't come from Google straight to deletion (possibly with a stop by “mark as spam” en route.)
First because it leaks the underlying email (you can safely assume all spammers are well aware of this feature).
Second because if you start receiving spam, you can't stop it. All you can do is to try to deal with the firehose.
A much better solution is randomly generated aliases that you can delete.
And you're confident of this how? I'm not actually convinced this is true. It's definitely a widespread belief though.
Adding a . in between any of the characters (or removing, if you registered the account to have .'s included) will still go to the same email address.
But you can't add .anystring to your address and still receive the message as you can with +anystring.
This always blows the mind of the average gmail user who thinks they registered first.last@gmail.com when they find out that firstlast@gmail.com also works.