I tend to do my yearly extention as soon as possible. In Chiang Mai that is 45 days in advance. My reason is you never know what might change. And as said the extention date still stays the same.
it starts to get scetchy if you have many back to back visa exemption entrys. How many? As Robert Lagas said it up to the discretion of the immigration officer at the entry point. Better get yourself the appropriate visa for a longer stay.