Beta distribution/PERT estimates

Hi
Which activities are best candidates for PERT estimates (O+4M+P)/6? Only the schedule activities on critical path or any schedule activity in project?

Regards,
Saeed

Sorry I should have added more information. I understand from previous responses from forums that PERT is recommended for critical path. Does this mean it is not allowed to use on non-critical paths?

If it is not allowed when what is recommended for non-critical path activities? Triangular distribution

Many thanks.