My personal opinion is that each contribution is assessed on it's merits and that the developer should strive to use the lowest practical version of PHP possible, but no lower. In some cases, that may mean the code is backward compatible to PHP 5.3. In other cases, PHP 5.5 may be required. I don't think you can set a hard version anymore because PHP doesn't move nearly as slowly as it once did.
Regards,
Andrew Eddie