[pstricks] Problems with uput and origin

[Manuel Toniato]

Hi there,
My name's Manuel and I'm an Italian student in Telecommunication Engeneering.
Yesterday in an Italian ML on latex, I have been adviced to use pstricks to draw all the graphs in my thesis.
So I began to use pstricks and I'm enthusiastic about this package.
However, I have some problems using uput* after changing origin.

This is the code I wrote to draw a Cartesian Plane.

\psset{xunit=5pt}
\psset{yunit=5pt}
\psset{dotsize=1pt}
\psset{origin={-15,-15}}
\psset{griddots=1}
\psset{subgriddiv=0}
\psset{gridlabels=0}
\psgrid(0,0)(-15,-15)(15,15)

\psline[linewidth=0.4pt]{->}(-16,0)(16,0) %z_1 axis
\uput*{5pt}[270](16,0)

\psline[linewidth=0.4pt]{->}(0,-16)(0,16) %z_2 axis
\uput*{5pt}[270](0,16)

The problem is that \uput* draw the white box in the right place but text is put using the old origin.
How can I solve this problem?

Thanks in advance!!
Manuel

P.S.
I have one more doubt. Why did I have to write \psset{origin={-15,-15}} instead of \psset{origin={15,15}}?