captaincancel: No, it doesn't matter if you start on the first beat of jammr's metronome or not. What matters for playing in sync is:
1. The tempo matches jammr's BPM value.
2. The number of beats of the loop/chord progression matches jammr's BPI value.
If this is counterintuitive, try this example to see why it's okay to start any time as long as you follow the above rules:
We're playing Am Am F G starting on the jammr metronome's 1st beat and everything is in sync. Now we decide to try something different. Let's play F G Am Am starting on jammr's 1st beat and this is in sync too.
Wait, I said F G Am Am on the 1st beat but that's equivalent to Am Am F G starting on jammr's 8th beat!
The answer is it doesn't matter whether we start on the 1st or the 8th beat (or even in between the metronome beat).