Its completely up to you but there is no harm in using Ajax programming for any website as they are used to increase the speed of website. You need not to implement it on every page but you can use it on pages which is having more hits or slower than other pages.

Hi Ajax, is a group of inter-related web development techniques used for creating interactive web applications.Using AJAX we can reduce the unnecessary hits to server/ database. So you can go for AJAX .

hi friend,
This is jagadhish.Frankly why people go Ajax means
1.With out page re-loading we can get the response.
2.For Asyncronus communication.

These are the reasons.
Thank you friend.