Is it considered safe to sonicate with flammable solvents?
In general it should be relatively safe considering you take all the precautions required when working with volatile flammable liquids.
Specifically for sonication I can think of two safety aspects:


Sonification promotes formation of the aerosol/fog consisting of the atmospheric gases and solvent vapors.
Depending on its composition, it might or might not be fire hazardous or even explosive.
That's why keeping an ultrasonic setup in a ventilated area — a fume hood, ideally — is a good idea.
Keep the power supply and all controller circuits outside, if possible, in order to prevent spark vapors ignition.  
Be careful when sonicating heterogeneous solutions.
Some materials would absorb the ultrasonic energy and this can result in local overheating.
To prevent this, use mechanical stirring.