<p>On a site where 90% of the pages use the same libraries, should you just load the libraries all the time or only load them when needed? The other pages would be ajax or simple pages that don't have any real functionality.</p>

<p>Also, should you only load the code when needed? If part way down a page you need a library, should you load it then or just load it at the top. Maybe it's possible it may never get there before of an error or wrong data. (Loading at the top makes it somewhat easier to understand, but may result in extra code not needed.)</p>

<p>I'm also wondering if I should make the libraries more specific so I'm not say loading the code to edit at the same time as viewing?</p>

<p>Basically, how much should I worry about loading code or not loading code?</p>

I would always try to give a file, class, and method a
single responsibility
. Because of that, separating the displaying from the editing code could be a good idea in either case.
As for loading libraries, I believe that the performance loss of including non required libraries could be quite irrelevant in a lot of cases. However,
```
include
```
,
```
require
```
,
```
include_once
```
, and
```
require_once
```
are relatively slow as they (obviously) access the file system. If the libraries you do not use on each occasion are quite big and usually include a lot of different files themselves, removing unnecessary includes could help reducing the time spent there. Nonetheless, this cost could also be reduced drastically by using an efficient caching system.
Given you are on PHP5 and your libraries are nicely split up into classes, you could leverage PHP's
auto loading functionality
which includes required classes as the PHP script needs them. That would pretty effectively avoid a lot of non used code to be included.
Finally, if you make any of those changes which could affect your website's performance, run some benchmarks and profile the gain or loss in performance. That way, you do not run into the risk of doing some possibly cool optimization which just costs too much time to fully implement or even degrades performance.