You STILL have to put 7 men on the line of scrimmage and 4 men behind it, and the eligible receivers are STILL the 4 receivers behind the line of scrimmages and each guy on the end of the line for the guys on the line of scrimmage.
If your center goes to the referee and tells him he's coming in as an eligible receiver, then lines up on the ball in the center of a standard looking line (with the guards and tackles to either side of him, therefore making him NOT on the end of the line), and he runs downfield, the offense will be penalized. I think it's most likely illegal receiver downfield, but it may be illegal formation.
Either way, the point is that whether or not you tell the referee you're eligible doesn't matter (except in the NFL). Whether or not you are eligible is based on pretty clear rules that are easy to understand.
The ONLY reason it is any different in the NFL is because there is a strict numbering system (to make it easier for the refs to keep track of who is and isn't an eligible receiver). However, an offense my have a set in which a guy with the number of an offensive linemen (ineligible receiver) is lining up as a receiver. NFL rules dictate that this player MUST let the officials know he is lining up as an NFL receiver. College and high school players tend to do the same (and it's announced on the PA, etc), but it's not required.
And as I said, no matter what anyone tells the referee, who is and isn't an eligible receiver is 100% based on how the offense is lined up when the ball is snapped.