Using:

• mixing live signal through pd
• effects
• MIDI-Synths

Normally you dont hear time differences below 10 ms on percussion stuff so I think this is enough.. But you have to do the mixing complete through pd (havin same latency on all signals, Microphone) otherwise there are comb filter effects, playing through the speakers.

Experience:

When using liveeffects and mixing through pd, it is very disorientating if you have a latency more than 20ms, just tested it and feeling not good having this to improvise.