I always climbout in WEP until the engine reaches normal operating temperature (beginning of yellow zone), where I disengage WEP. Exceptions are the various Lavochkins, 109s and 190s. I leave these in WEP until the temp needle is 1/3 of the way up the yellow scale. This way, I always have adequate WEP reserve should I need it.
There is absolutely no reason NOT to climbout using WEP, so long as you disengage as described above. Any increase in speed and/or altitude is beneficial.
My regards,
Widewing