Do small preamp tubes 12ax7 need to be matched?

Ideally yes but you don’t need to be too OCD - with small triodes, often you’re trying to match 4 elements (2 triodes per tube) so it can be frustrating to try to find 4 within 10% of each other on the major parameters (mu, transconductance) - especially vintage tubes. And even up to a 20% difference sounds like a lot, but it can be just fine for many applications. If your elements are within 15% (max) of each other you’re generally doing quite well. I believe Upscale matches their "Platinum" grade to within 10%.

If you have an egregious mismatch, well over 25%, then that can start to cause very audible issues. I had a pair of Cifte 6189 silver plates where one tube was much stronger than the other, and it caused about a 1 dB L/R channel imbalance when used in the mu follower slots of my phono stage.

Once I also bought a trio (pair and a spare) of Brimar CV4004 (12ax7) from Parts Connexion, unaware that they were going to be COMPLETELY unmatched from an extremely variable stock. They were a complete mess, egregiously mismatched on triodes and tubes (variances or 40% and more), and sounded like an absolute mess as a phase splitter my tube amps. Fortunately the vast majority of dual-triode tubes that have been sold to me as "matched pairs", over the years, have been as advertised and sound symmetric when in an amp. Parts Connexion is the only shop/dealer I'll never buy tubes from again. 

Some of the old tubes were abused by gear, more on one side than the other. I’ve seen a number of Tung-Sol BG RP 6SN7 (the holy grail tube) where one triode measures like new and the other is practically dead (measuring 50% less than the strong triode). Those are no good, so watch for that :(

Some circuit slots are going to be more tolerant of ax7 / at7 / au7 / ay7 type swaps than others. Just depends on the circuit. Cathode followers and mu followers tend to be pretty tolerant, though 12ax7 probably aren’t the best choice for driving a line of cables & SS amps.

The Herron phono supports at7 / ax7 swaps in 2 slots to change gain +/- 5dB or so. The Rogue phono is fine with at7 or au7 in the mu follower slots (similar gain difference). If you change the type in the RIAA / MM ax7 slots then you’re going to get something that sounds very much not as intended, and not accurate, though 5751 are typically a "close enough" sub for ax7.

I once accidentally put 5814 / au7 in the V1 / ax7 / phase inverter slots of tube monoblocks (Rogue Apollo) and it actually sounded pretty good, but certainly very different and colored before I noticed the reason. It was a pleasing type of coloration, actually.

If a type sub changes the sound a lot, that’s usually a coloration because the circuit isn’t designed for that tube type. One exception to this would be using a 12bh7 in place of the stock 12au7, which usually sounds a lot better (amp must be able to provide the extra heater current), because 12au7 sucks and the 12bh7 does what it does, just better.