First off what is it that you register for?, PESEL or Karta Pobytu?
You need Karta Pobytu to legalise your stay. You are EU citizen with intention to work in the country, so in order to stay here legally more than 6 months, you should obtain the paper. The foreigner from EU without this document, after 6 month period, should demonstrate that he continues to actively seek employment and has a real chance for it.
PESEL
The PESEL number is mandatory for all permanent residents of Poland and for temporary residents living in Poland for over 2 months.
en.wikipedia.org/wiki/PESEL
When you work in Poland - granting PESEL number is done at the written request of the taxpayer (eg. the employer);