FlapsOne has it right in john_tullamarine's link.
Average power for a modern Wx Radar as fitted in the B747-400 is around 24 Watts and the minimum safe distance on transmit is 14 to 15 feet. That is not true of the 'toasters' fitted in older machines, some of them, like the old Bendix unit that avioniker mentions above, can produce dangerous radiation for hundreds of feet ahead. There's not many of those older units left nowadays though and low power radars have been more or less standard BFE in large transport aircraft built since the late seventies and early eighties.
Military interception radars are another story, and many can cause the effects described in the link by Lu Zuckerman, but then they are airborne search radars, with concentrated high power beams tuned to detect other aircraft.
The answer is, its safe to turn on your Wx Radar during taxi if it is a modern digital low power unit. If you don't know what you've got, ask to consult one of your company's avionics technicians.