If you are buying regular tickets (not AP's) you can get them cheaper by buying them before you go. Ticketmania is the best in my opinion. They give you a discounted price and ship them for free. Also, if you go to mousesavers.com and sign up for the free newsletter, she has a link to ticketmania that will get you the tickets for even less. They are great to work with. We have ordered ours through them for the past several years and have never had a problem. You get the tickets in record time. This time, I ordered the wrong tickets (forgot to add the hopper part) and they let me send them back and they are issuing me the new ones at the mousesavers discounted rate and they are shipping them again for free.
For AP's, it really doesn't matter. They are the same before you go or when you get there.
For example----I went on the disney site and put in a 7 day ticket with park hopper and magic plus. The price is 285.44--this includes tax. It said that I was saving $11 by ordering early--that is on one ticket. Then I went to the Ticketmania site and the price of the same ticket is $279(includes tax)--saving $6.44 more. That is total savings of $17.44 per ticket. That can really add up when you are ordering tickets for a whole family.