IMHO it doesn't hurt to write them all. The actual Senators or Representatives are probably not even going to read them anyway so it is okay to send the same letter to each person. If you were looking to cut down on how many letters you need to send here are the important ones:
Sen. Allard - he voted against the ban last time but he has higher political aspirations and is trying not to look "radical". We need to keep the pressure on him.
Rep. Scott McInnis 3rd District - voted against last ban but not a die hard supporter of gun rights.
Rep. Tom Tancredo 6th District - supports high capacity magazine bans, trigger locks etc, keep the pressure on him.
Rep. Bob Beauprez 7th District - new congressman that won by a fraction of the vote. He needs to know where his public stands so the letters are very important.
Probably will not vote for a reauthorization:
Rep. Marilyn Musgrave 4th District - she will vote against the ban but it would be nice to write her a letter of support.
Rep. Joel Hefley 5th District - voted against ban and represents one of the most conservative districts in America, write him in support anyway.
Probably will sign it if given opprotunity:
Sen. Campbell - voted for ban last time, and may not run again.
Will actively sponsor legislation:
Rep. Dianna Degette 1st District - waste of time.
Rep. Mark Udall 2nd District - waste of time.
Above is roughly stolen from RMGO.
Of course write the Senator and Representative that need your vote, the other ones probably don't really care what you think since you are not in their district and can not vote for them but what the hell. I like the feeling of dropping a stack of envelopes into the mailbox, just as much of a freedom as buying a gun.