I find that good tires can hold pressure perfectly for many months with nothing but good old 80% nitrogen air in them. I also find that some are less than perfect, but you have to keep checking them to be able to remember which is which.
I keep all of my tires at pressures near the number on the sidewall, because it provides both the best tread wear and the best gas mileage. It also gives you more time to discover the problem on a slow leaker.