It looks like about 70% of unspayed dogs will develop mammary masses after age 7 and 50% of those will be malignant.
http://www.caps-web.org/dr_allen_com...mal_cancer.php http://www.petconnection.com/blog/20...e-science-say/
I can understand your concern about anesthesia, but complications really aren't that common. Make sure you get pre op bloodwork and a bile acids tets done before, though, to alert your vet to any possible issues that could cause a problem.