When talking about neutrals in relation to color we must always remember we are still talking about color. There was an old academic saying the grays make the painting. While this is something I believe is true it is something that can get out of hand if the proper placement of an almost neutral color is not carefully considered.
Some artists like Remington used to save his color scrapings because they ended up a nice green gray and were useful in lieu of a tube gray. Tube grays have their place and can help with colors like yellow which can be difficult when you are trying to only lessen its chroma without changing its hue or value.
Having said that I still think it is better to mix your grays. You have one less tube of paint to lug around or buy and will avoid the trap of overusing the tube gray. If you’re like me when you put it on your palette you know you will not want to waste it, this is how inappropriate gray ends up in all your colors.
If you choose to mix your grays you will also get a more personal interpretation and it is possible to find other color solutions that while close to neutral are just low chroma color. It is slower to mix and that should be weighed against ease of use for a tube color.
In my observations nothing in nature is pure gray. All neutrals are affected by some other color around it from direct light or indirect light. This is especially true outdoors. Whatever you choose; tube color or mixing, the subtle beauty of neutrals will enhance your paintings.