GET vs. POST does it really really matter?

According to the HTTP RFC, GET should not have any side-effects, while POST may have side-effects.

The most basic example of this is that GET is not appropriate for anything like a purchase-transaction or posting an article to a blog, while POST is appropriate for actions-that-have-consequences.

By the RFC, you can hold a user responsible for actions done by POST (such as a purchase), but not for GET actions. 'Bots always use GET for this reason.

From the RFC 2616, 9.1.1:

9.1.1 Safe Methods

Implementors should be aware that the software represents the user in
their interactions over the Internet, and should be careful to allow the user to be aware of any actions they might take which may have an
unexpected significance to themselves or others.

In particular, the convention has been established that the GET and
HEAD methods SHOULD NOT have the significance of taking an action
other than retrieval. These methods ought to be considered "safe". This allows user agents to represent other methods, such as POST, PUT and DELETE, in a special way, so that the user is made aware of the fact that a possibly unsafe action is being requested.

Naturally, it is not possible to ensure that the server does not
generate side-effects as a result of performing a GET request; in fact, some dynamic resources consider that a feature. The important distinction here is that the user did not request the side-effects, so therefore cannot be held accountable for them.

It does if a search engine is crawling the page, since they will be making GET requests but not POST. Say you have a link on your page:

http://www.example.com/items.aspx?id=5&mode=delete

Without some sort of authorization check performed before the delete, it's possible that Googlebot could come in and delete items from your page.

Since you're the one writing the server software (presumably), then it cares if you tell it to care. If you handle POST and GET data identically, then no, it doesn't.

However, the browser definitely cares. Refreshing or clicking back to a page you got as a response to a POST pops up the little "Are you sure you want to submit data again" prompt, for example.