That's the beauty of the method I use. Nobody sees the name+xyz@gmail account. In fact, they don't see Gmail at all. They see email@example.com which is forwarded to firstname.lastname@example.org behind the scenes.
And with my host's control panel, it's not hard to mass edit all my forward-only e-mail accounts to change the forwarding address if my Gmail account finally is overrun with spam.
But let's put it this way: I've been using this method with my current Gmail account, which I've had for over two years now, and the only spam I ever get is an occasional newsletter from Xara, who just won't remove me from their mailing list.
But what do you do if you DID want to reply to the sender (it wasn't spam)? Then they would know your address.
That is why I use www.spamex.com
which for $10/year gives me 500 disposable addresses. The emails are all forwarded to a real email address somewhere. And if I reply to someone, they massage the header, so the addresses I reply from doesn't show up. I can also send attachments up to 500k. I have a bookmarklet on my links bar for when I want to bring up spamex and create a new address or access the interface. Since the domain name contains the word spam, I have had some occasional problems with some ISP's refusing to forward mail to it. Which is why spamex also allows the use of the xemaps.com name (spamex backwards)
I use spamex for all forum registrations, newsletters, etc.
I also have my own domain name where I have 2 addresses that goes to Outlook. Only very trustworthy friends/people/companies get these addresses. I hosted this through www.addaction.com
because I can use my own domain name directly in outlook (send/receive) or I can use their email servers (which is what I use since they allow me to use an encrypted connection, port 995).
I have other email accounts also, such as 2 gmail accounts (Gmail has been very reliable but since Google scans all email looking for keywords to generate advertising for you, one wonders just how secure gmail really is), a Yahoo account (never use, always full of spam), an inbox.com account (hardly use but can send 50mb attachments), mail.com (hardly use) and an account at safe-mail.net (this is where spamex feeds to. This account is always HTTPS but the free version only provides 3mb of storage. However, I've gotten maybe 3 spam emails here in maybe 4 years of use). Probably a few others lying around also...