If you haven't already you should read the migration guide with particular focus on Backward Incompatible Changes and Removed Extensions.
You have bigger issues than deprecation. Ignoring E_DEPRECATED will not suffice. Because of the incompatible changes there will also be other type of errors or, maybe, even worse, unexpected behaviors.
Here's a simple example:
<?php
function goto($line){
echo $line;
}
goto(7);
?>
This code will work fine and output 7 in PHP 5.2.x but will give you a parse error in PHP 5.3.x.
What you need to do is take each item in that guide and check your code and update where needed. To make this faster you could ignore the deprecated functionality in a first phase and just disable error reporting for E_DEPRECATED, but you can't assume that you will only get some harmless warnings when porting to another major PHP branch.
Also don't forget about your hack and fix the deprecated issues as soon as possible.
Regards,
Alin
Note: I tried to answer the question from a practical point of view, so please don't tell me that ignoring warnings is bad. I know that, but I also know that time is not an infinite resource.