Do small preamp tubes 12ax7 need to be matched?

Matched tubes will always give better sound.

You can "use" a 12AT7 to replace a 12AX7, in that the pin connections of the two tube types are identical, but that would be far from ideal.  The 12AT7 has less gain, a lower plate resistance, and a higher transconductance compared to a 12AX7.  In a phono stage, the difference in gain would be quite noticeable, even with high output cartridges, if one were to replace a 12AX7 at the phono input with a 12AT7.  But also the 12AT7 would function suboptimally in a circuit designed for a 12AX7, so there might also be noticeable distortion.  (I am qualifying my statements, because I know someone is going to claim that he used a 12AT7 in this fashion and it sounded wonderful.)

Mulveling mentions encountering 6SN7s where one triode tests like new and the other triode tests like its near death.  (The 6SN7 is a dual triode, with two completely independent triodes in a single glass envelope.) This can happen with a tube that has been pulled from a piece of equipment where only one of the two triodes (the dead one) was actually attached to the circuit.   Often equipment was built this way so that one could swap L and R channel tubes after the tubes seemed to need replacing, and thereby gain use of the unused section in the opposite channel.  

I am not a big believer in paying a premium for matched tubes, because tubes do not age in synchrony.  So after a few months of use, one of those two tubes may test quite differently from the other.

Lewm brings up a very good point when he states that tubes age at different rates. I have always wondered how tube matching makes a lot of sense when a) the tubes will age at different rates b) what are the parameters that one can say a tube is matched to another? c) your circuit is either driving a tube hard or it is not-- which results in varying the life and the point in (a) and lastly d) noise and distortion vary greatly by tube design and placement in the circuit.
Anyone here truly know what a tube tester is accurately testing for?

You can "use" a 12AT7 to replace a 12AX7, in that the pin connections of the two tube types are identical, but that would be far from ideal. The 12AT7 has less gain, a lower plate resistance, and a higher transconductance compared to a 12AX7. In a phono stage, the difference in gain would be quite noticeable, even with high output cartridges, if one were to replace a 12AX7 at the phono input with a 12AT7. But also the 12AT7 would function suboptimally in a circuit designed for a 12AX7, so there might also be noticeable distortion. (I am qualifying my statements, because I know someone is going to claim that he used a 12AT7 in this fashion and it sounded wonderful.)

I know it works perfectly and a person who replaced 12AX7 with 12AT7 in his new Mac preamp noticed huge upgrade in sound and (no problem with gain). They are in use ever since for more than one year for everyday listening sessions as far as I know. German military NOS telefunken are superior to any Russian tubes. My advice to OP is to stay with NOS Telefunken tubes (...ax7 or ...at7), matched pairs!  

"I know it works perfectly."  It just might in this ONE case you site.   The two tubes are not close to each other at all.  12at7 gain 60 12ax7  gain 100
Say they are run at 250 volts B+   12ax7  plate current  1.2 ma  12at7  10ma 
Plate resistance 12at7  10900 ohms 12ax7  62500 ohms.  Grid voltage 12at7 -12  volts 12ax7  -2  volts.  Etc...  Lewm is on the money these two tubes, short of the heater current and can be run on the same b+ , are not even close.

It would be advisable  to have a schematic and know the size of the transformers and how much current the power supply can handle to know if there could be a problem  with the parts.  There is some math to do to set the tubes where load lines are ideal.   

That being said I prefer the sound of 12at7 over  12ax7 in phono stages.  But would design it accordingly.   If the guy who has the Mac preamp actually  figured out where to run the 12 at7  properly  it could even sound  better.

Enjoy the ride
Tom

PS.     In regards to matched tubes ,over rated in my book.  Plus short  of having a load  line tester , tube testers are ballpark at best.  Also the op could use 2 different  types he has in on hand one on each side see how much difference he hears even with  two different  brands.