I get asked this question a lot, short answer … Yes. But there is a reason that very few contractors use redwood over pressure treated (or pt) for mud sills and bottom plates.
Let me first say that I have remodeled homes in areas of San Francisco and Marin that were over 100 years old- built entirely out of redwood- and are still in great shape structurally. That is because redwood is naturally high in acids which causes rot and keeps bugs from eating it, seeing as those are a homes two worst enemies, redwood makes good sense. Until you look at the price tag. Fact is many redwood areas were over logged and the ones that were not now protected. As a result the price of redwood is very high compared to other framing products and they are now using younger growth trees which equals more knots, twists, and wanes.
So to cut cost Pressure treated lumber was introduced as a wood that could mimic the attributes of redwood but for half the cost. Pressure treated lumber is usually Douglas fir or sometimes hem fir and injected with many toxic chemicals (copper or zinc napathane being the most potent). These chemicals are what prevent the rot and pest infestations but have also been known to cause cancer by polluting ground water when disposed in land fills. In California, the scrap pieces of pt must be disposed of at a bio-hazard waste yard. Recent the levels of toxins have been lowered by approx. 50%.
It is also worth noting that it is never acceptable to attach un-treated Douglas fir lumber in direct contact with concrete. There are several ways to treat Douglas fir yourself and make it rot and pest resistant. The # 1 way to do this is apply copper napathane in either a spray can or regular liquid to be brushed on. You can find this at just about any neighborhood hardware store but be careful doing this! It will stink bad for a day or two so do not apply directly in living spaces. Be sure to read any and all factory directions regarding the application of any sealant or rot preventing product.
So to answer the question, yes … Absolutely, redwood is a great product for mud sills. But if you are gonna use redwood, save yourself some money and get 'rough cut' lumber, this will be slightly larger so you'll have to cut it width wise, but the price difference is worth the extra work.