Safety would be the number one reason. It just lets all know that the play is dead and prevents possible problems and injuries.
The thread in refstripes has some good points from other officials. It is at least something to discuss IMO.

The only problem is the whistle you may be mirroring may not be from another official.

It's an exercise, Jim. Nobody ever suggested no using a whistle in a game. Besides, what would you do with your lanyard if you didn't have a whistle?

What you thought was a whistle may not have been a whistle. (Or it may have come from the stands, or a coach, or a plane flying overhead, etc.)

If one member of the crew has an IW, he/she should be the one to 'fess up and blow the whistle again.

In our local association we tell ALL our officials, especially the new ones, unless you see the player down with the ball in his possession, do not blow the whistle. We also teach our officials to only signal a TD if it is to their side of the field, and they clearly see the TD.

To sum it up, see the ball, make the call. If you don't see the ball, don't make the call.

As for your original question regarding advice, don't let the crowd bother you. The best thing for you to do is pretend they are not there. The only call that matters is yours and that of the other officials. Not the few people in the crowd who apparently have a rule book at home.

Here are my thoughts, FWIW:
Don't mirror signals.
Don't mirror whistles.
Don't double up flags.
Keep the whistle out of your mouth, IF you are comfortable with it. If not, BE SURE you see the ball prior to blowing the thing.
There is no harm in having a play without a whistle.
On an incomplete pass, there is no harm in blowing a short whistle to signal to everyone else that the play is dead.
There is no harm in having no whistles on a "three yards and a cloud of dust" type running play.
There should never be more than two whistles on any given play. If there are more, too many eyes on the ball.

Just my opinions.

I pretty much agree with everything you said. Except I stongly suggest that if you are not comfortable with the whistle out of your mouth, you better get comfortable with it. Not only does it prevent IWs, but it also saves you a trip to the dentist.

I'd agree with everything grantsrc said aprt from maybe the "dont double up flags". Not sure exactly what grant meant by that. If by "doubling up" he means 2 officials flagging the same foul because they both saw it, then I'd disagree with him saying dont do it.

Never assume the other official saw the same foul, I say flag what you see. If it is there, two flags sell it better to the players and coaches.

I'd quantify that by saying that if a foul is well outside your area, then (1) what are you doing seeing it in the first place, maybe you should be watching elsewhere and (2) you'd better be 150% sure of what you saw and if you are doing this more than once or twice a season then you are gonna get up the nose of your colleagues.

I have had an occasion where I saw a foul and flagged it and saw another official's flag come in on the same yard line. I started going toward him to double-check he had the same foul as I did. He picked up his flag and backed off and waved me toward the Referee to rport the foul.
I reported my foul, but when the Umpire started marching it off, this other official came in telling us we were marching the wrong way. His foul WAS NOT the same as mine. So the referee now had to redo the whole enforcement, talk to captains etc. made us look like idiots. A learning experience and nowadays I always check with the other guy that we did in fact see the same thing before one of us goes to report it to the Referee.

Remember the answer to #61 on the Part I NFHS is False!
 

All players who are ELIGIBLE at the start of the down REMAIN ELIBIGLE throughout the down

What I meant was don't throw your flag just to support your fellow official, if you don't see it. Some do this in order to "add" credibility or support to a call.

Totally agree with you there grantsrc. Hadn't hear the term "Double up". Over here, we call it a "Me too" flag. When I'm white hat, I hate asking a guy what he's got, only to told "I've got the same as him" :(

REPLY: Not sure what you're responding to here, but remember that some officials reading this board work under NCAA rules (especially our European brethren) where your statement is definitely not true. I know you mentioned Fed in the subject line, but some might have missed it.