Hate writing long reply's only to be asked to re log in and loose everything I've wrote!
Back to topic, it's been said many times but once again- always cap off pipe work! You wouldn't get away with leaving a valve shut without capping off a gas pipe!! So it's good practice to just always cap off.
I'd say your boss sounds like he's getting a little desperate reprimanding all the boys for leaks, but then it also sounds like you get more than your fair share! 🙂
Your boss should train you properly if he intends on putting you in charge of jobs, and if he's so upset over the leaks he really should implement end of job check sheet, I always pressure test my pipe work before filling up, it'll show up the smallest leak and save money on return visits to fix leaks, it might add a little more work but far less than returning to fix leaks!!
We all get the odd leak buddy, maybe not as many as you whilst still learning the ropes, but it's not getting leaks that's makes you a bad or good plumber, it's finding and dealing with the problems before you flood a property!
Belt and braces, insist on doing a pressure test on your work before you fill up, it's not an admission of fear of leaving leaks, it's being professional and caring about your work.
Again though, that said- if your getting leaks on ball valve connections and compression fittings ect maybe you could do with going back to basics on the training, you'd be surprised how many newbies I've trained who think the tighter you do up compression fittings the better........
I hope all goes well for you, stand your ground, in this case the guy who did the filling should be responsible in my opinion.
Let us know how it goes.