Id actualy say the opposite. If youre just starting out you are best sticking to penalty area width and length of the pitch. Keep it as simple as possible - and follow the play. Positioning of credability so that you are close enough to both see the foul and then when you blow and erveryone turns around, you are stood right on them - that is worth more than anything.
Offside - from where you are, does it look like they are too close or have too much of a lead over the defenders to have been onside. One thing that you will learn quickly is that everyone will always call for the OS, and the reaction to what you do, or dont do, will tell if you were right or wrong.
If everyone shouts, they get the ball and someone is genuinely losing the plot at you not blowing, you can alwasy blow and just that you were blowing late.
On the other hand, if you decide first and blow up - if the attacker spins, glares and probably makes his thoughts know (and the defenders stay quiet) then you might have got it wrong.
I talk to the teams before KO, and i tell them that I've got no assistants, so some i will get right, some might be less right, but I will do my best. That way, I've lready said my piece and need say no more during the game.
But whatever you do, do it confidantly, and be true to yourself. You think he was off - blow that he was off, "offside thank you" and moe on to the next phase of play.
What is it they say - if you're not 100% you cant give it ....