Check That Liner First

Jason G - Is it acceptable to use .045 wire with a 1/16th contact tip?

Jason,
That's not a "code" question, that's a "weld process" question.
My answer:  You should not have to and there is good reason not to.
The contact tip is the location where current from the electrode lead is going to transfer to the actual electrode (wire).  The larger diameter the hole the wire is passing through, the poorer the transfer.
Of course, a contact tip that is too small a diameter will lead to feed problems, but it would not be common for a contact tip manufacturer to manufacture 0.045 inch diameter contact tips with too small of hole.
You may find yourself experiencing feed or burn-back issues and thinking that increasing tip diameter should solve your problems.  Doing so may help minimize a symptom, but you have not corrected your core problem.
The first place I'd look for solving THIS problem is the gun liner installation.  Gun liners, when installed, need to rest tight against the defuser (part holding the contact tip).  Many mistakes can be made when installing a liner.  One of the biggies I see, is cutting it too short.  A short liner leaves a space between the end of the liner and the back of the defuser.  This space will allow the wire to wobble before entering the contact tip, leading to burn back.  It will also leave space for the buildup of metal shavings which can eventually become the "point of transfer" and lead to burn back.
Someone who does not understand the proper technique for installing a liner will change the liner once problems develop and then not realize they have just set themselves up for failure and frustration.
Always follow the gun manufacturer’s instructions and before cutting it to its final length keep these tips in mind:
1 - Keep the gun as straight as possible.
2 - Mount the gun securely onto the feeder.
3 - Calculate the length the liner needs to extend past the end of the gun (often the gun manufacturer will list this dimension).
4 - **MOST IMPORTANT** Before cutting the liner at that dimension, apply pressure pushing the liner into the gun (you'll find the liner will easily move 1/8" - 3/16"), hold that pressure, measure and cut.
5 - **EQUALLY IMPORTANT** File down the bur formed by cutting.
Give that a try.
PWC