It matters a little because you lose power due to the mismatch.
In principle, you lose 5.7 dB when transforming 50 to 75 Ohms or vice versa. While not a lot, that is enough to make a noticeable difference with a small antenna or poor receiver. With a Beverage and a good receiver, no problem.
RG58 is the 50 Ohm standard and is super widely available.
But the most important reason to use 75 Ohm RF6 0 to me - is that the RG6 crimp connectors are waterproof.
Chuck
http://www.hard-core-dx.com/article.php?story=20190128061505715