Just a couple more thoughts on this issue:
1) Getting enough sunshine per day might be doable in San Francisco, but it certainly is not possible where I live, in the Netherlands! Even if the sun is shining for us, it often isn't strong enough to help with vitamin D production in the skin (despite our fair skin adaptation!). Therefore, Dutch doctors recommend all citizens - independent of diet - to supplement vitamin D during the months that the sun is not strong enough, which boils down to months that contain an "r" in their name (i.e. from early September to late April). In addition, vitamin D is added to all dairy products and dairy substitutes.
2) Knowing your vitamin D levels is only part of the story, if you have symptoms that could signal some deficiencies you might also want to know your serum calcium, your phosphate and magnesium levels, maybe even your parathyroid hormone levels, because they are all related. Knowing the whole picture is more informative and can provide a better answer as to whether to supplement vitamin D or not. In general though, if you have no symptoms, don't even consider checking your vitamin D status. Why would you want to know if you don't want to treat it anyway if it turns out to be low?
3) What is regarded as "too low" is very dependent on a populations normalized values. Just to give an example, a cholesterol of 100-120 would be very low in the USA, but not in rural China. In other words, these values are very relative. As already mentioned by "TominTN" earlier: "The recommendation for combating this trend, as I understand it, is to stop looking so closely at the numbers for individual markers and pay attention to whether the body is manifesting disease." Very wise words.
4) I can recommend to keep a daily diary of how you feel in general. If there is a downward trend over a substantially long period of time, start looking for ways to reverse that trend. You don't have to be technically ill to work on optimizing your health! Who knows, maybe supplementing vitamin D, vitamin B12, or omega-3 fatty acids (preferably in the form of algae-derived EPA/DHA) for a while brings you back up to a more optimal health level.