I find reality checks very useful, a lot of my lucids start with them. I've been doing this since I can remember and been lucid dreaming for plenty of years now. The way i see it is that they are focusing your memory to stay alert, a bit like when you go shopping without a shopping list, there's that bit of your mind that is constantly active waiting to see the thing you need to remember to buy. Mindfulness is handy but unless you're some kind of radioactive zen monk, you're still going to need a few mind tricks to keep your awareness ticking away. I don't think it's and either/or choice anyway, reality checks are more than just mindfulness they are a physical act that snaps you out of the moment and gives you a nice way to check for certain what reality you're in. Plenty of times i've done them and been surprised to find I am dreaming.