Atm i have a concession myki but its the one where you just top up whenever you're low on credit. Is this the one you are talking about b^3?
Yes and no, when you go to top up select Myki pass instead of Myki money (well I think you have the same one as me...).
If you're using myki money, you should touch-off, or you'll be charged a default fare. If you're using a myki pass, like a weekly or a yearly, you don't need to touch-off (but it doesn't matter if you do, it doesn't change anything).
This is what you should be doing, although for a few weeks around exams when I was using myki money instead of myki pass I didn't touch off... because the distance I was travelling was going to charge me the full fare regardless... (zone 1+2 twice, going from one side of the city to the other e.t.c) so I got lazy, and well soccerboi would be in the same situation there. But otherwise do what Shenz0r said.
I've encountered several officers but no one asked that thing...that's why I find it unnecessary..why not two more extra trips? For default fare, is it a dollar and something? Thats what costs me for a single trip
As of lately they've been cracking down on it more. Even had inspectors dressed up as a group of bikies one day.... There also was a period during semester 2 last year where I got checked 6 times within the 5 day week to uni, 4 on the way in and 2 on the way home.
They can only tell if they are looking at the myki gate as you touch on, as it shows what kind of myki you are using. Then they may stop you and ask for it.
I've been asked several times for my concession proof card 
I'm sure they can see what myki you are using on the scanner? and plus it has the big C on the card, when there are inspectors waiting outside the gates at Melb Central, I've had them see the card before I even went through and even nearly ask me before I got through the gate... or maybe I was just dodgy looking that day

I didn't know about this concession card thing. I got "caught" by a ticket inspector as well 
They just ask for a reason, then they'll send it to the transport department and they said that you'll hear from them in around 6 weeks.. If you don't then you've gotten away with it.
I forgot my full fare card so I took my brothers one, topped it up properly and touched on touched off what ever. Yet they even think of fining me
It'd make more sense if they asked me to pay the full fare which is understandable and makes sense.
Same thing happened to someone on my train at the end of last semester. They rang his mum on the spot to confirm address and such, sent him the fine but told him that he wouldn't have to pay it if he got the card and proved it back to Metro later on. Conicidentally he turned up to the physics lecture I was going to late on that day.
A guess it depends on the ticket inspector sometimes. I've seen them get on at one stop from the end of the line and catch 2 people for not having their transport cards on them. While I have also seen a guy get let off because he had to run for the bus, and only had a 50 on him, so the bus driver let him on. He then had to run for the train when he got off and so didn't top up (this was the story he told). We were again one stop from the end of the line when the ticket inspectors got to our carriage and they took his ID card or license or whatever and made him go up and buy a ticket, and then bring it back and show him, he did it and they let him off.
Then there was also the time where all of the scanners at Huntungdale were broken, and the train had just arrived and we all had to bolt up the ramp to get onto the train. I didn't get touched on, neither did half the others that got on. But I ran to a carriage further up... and as soon as I walked on I pretty much bumped into the ticket inspectors (who had already checked the other carriage that everyone else got on). Had a 'calm argument with them for 10 minutes, which at one stage I had two of them talking to me with one repeating the phrase 'not touched on, no fare'. I pretty much asked them what I could have done instead and well since I showed my concession carrd straight away and told them that it wasn't touched on before they scanned it, I got off... just... it wasn't as if I could have done anything anyways. Just lucky that the second inspector walked off and I got the reasonable one.