Good article so wont add to it.
Power Tubes:
Some amps are designed to require power tubes that have the same cathode current. Sometimes severely mismatched tubes will cause a hum and even some from the same manufacturer really need to be matched. It doesnt hurt to have them matched.
Re: pre-amp tubes, to quote another source,
TheTubeStore
Circuits that actually require matched pre-amp tubes are so uncommon that we only offer this service on an order by order basis. The price for pre-amp tube matching is $5 per tube and not refundable. Power tubes require matching because the way they amplify sound waves can create exceptionally bad sound if they are poorly matched, but pre-amp tubes generally aren't used this way and so matching them is unnecessary.