While not a 'native English speaker' I will chime in. I am a firm believer that as a visitor to a country it is a sign of respect to greet people and thank people in their own language. I travel around the world about 300 days a year and do take the time to learn the basics of language and custom.
In France in particular it is important to greet people with 'Bonjour Madame', etc before conducting your business, as well as to thank them before departing. This is taken very seriously and if you omit the greeting you won't get off to a good start.
Learning 'hello, goodbye, thank you, pardon me, excuse me' don't take very long, but will go a long way towards showing your respect for your hosts.
As to the amount of English spoken, I can only go by what others have written here as I am fairly fluent in French and thus conduct transactions in the language. In DLRP and major tourist sites you will most likely find English speakers, but at the metro ticket window you may not. The same for shops - if you are at one of les Grands Magasins chances are that the clerk will speak some English, but go into a smaller shop or bakery and the chances are far less.
Even if you don't speak it well, the fact that you have made an attempt to learn the basic courtesies will be appreciated.