Maybe, but most ISPs are lazy/cheap and can't do a full-take packet capture of all customers data at the same time. The ones that I have seen usually have a custom or logging DNS server that associates each domain request with a customer account. So yes, in many cases, changing your DNS server is enough to avoid the larger DNS sniffing operations. You should also use an IP check query to make sure that you are really using the DNS server you think, and that you're not being DNATed back to your ISP's DNS server.