A good filter (0.2 micron or less) filters out everything except viruses. Most of us have been vaccinated against the really nasty waterborne virus diseases, such as polio and hepatitis. However, there are the noroviruses, which won't kill you (unless you're immune compromised) but will make you feel as though they are! If you are upstream of habitations (septic tanks and sewers), you don't have to worry about viruses. If you are downstream of such things, add a chlorine dioxide tablet (such as Katadyn MicroPur), which will take care of the viruses in 15-20 minutes (the reason for the 4-hour wait time on the tablet label is to kill the giardia/crypto cysts, which the filter will already have taken care of). This is by far the cheapest/lightest method of double-treating.

You have another problem if if you are downstream of mine tailings (you know, those picturesque old holes in the ground), or of areas of considerable agricultural chemical use. Fortunately, most mine shafts are shown on USGS topographical maps. Unfortunately, there are no filters that will effectively filter out things like mercury and pesticides.

A final problem is if your water source comes from glaciers and is full of fine silt. That will plug up your filter in a hurry. In that case you're better off to use chlorine dioxide and (for really cold water) wait the 4 hours. You can get some of the silt out with a bandanna or coffee filter, but not enough to save your filter from sudden death!

For the Cascades, unless you are downstream from ranches/farms/towns, a good filter is sufficient. I always take some ClO2 tablets, though, just in case something happens to the filter, since they are so light in weight.

The best source on effective backcountry water treatment methods is the Centers for Disease Control. I do not consider authors of backpacking books, most of whom are not scientists, a reliable source of water treatment information!