I've used douglas fir as well. But I found that it does not have the same suppleness as spruce or bass. The extra brittlness led to problems when using it in areas which needed to withstand some shocks.... namely control line combat models...
While bass does not have the strong fiber content of spruce I'd say that it's not a bad substitute for spruce other than the smaller sizes. With those you'd want to slightly increase the depth so that handling the model won't result in the stringers being permanently bent inwards. For example if the design calls for 1/89 sq spruce for a stringer going with 1/8 x 3/16 bass should provide the same resistance to being deformed inwards. Alternately do not discount the idea of going with something like 1/8 x 1/4 very hard balsa. The harder and heavier grades of balsa are actually very strong. And a stringer of this size would easily withstand the same loads as the 1/8 sq spruce.