How to decide circuit's biasing (over temperature variation)

Dear all,

I got some questions for Biasing design. Can someone help me out?

Do we always use PTAT biasing current to bias ALL KINDs of circuits?
How to choose the slope (I vs temp)? Is there any rule of thumb for that slope?

How about biasing voltage? Should we keep it constant, or positive(or negative) propotional to the temperature variation?

Thanks very much...