As far as I understand the upcoming (damned by almost all) EASA FCL.1, having passed the CPL theory test (and holding FI on your PPL) you are allowed to accept reward (including money) for FI. I always though this was the case under JAA (but never actually read it). The proposal to ditch the CPL theory requirements for FI disappeared in EASA's proposal as it stands now. Whenever it makes sense to do CPL training once you have done the theory narrows down to job opportunities (and will ;-) )