Hi,

Is it too early to get their signature on the PR application papers?. For parent's sponsorship approval at least 2-1/2 or three years will be required as of now. Meanwhile, who knows, there could be (?) some changes too.
However, if you wish, you could keep application papers ready now -
undated signatures can be obtained
without attaching their photos. The PR application should be submitted at thier country of residence- (respective visa office - when sponsorship is approved. That time send the completed papers to the visa office with date and recent photographs.