How widely is English understood/spoken in Germany?

I am travelling to Germany next year as a tourist.  I am doing Rocket German partly for personal interest and partly because I like to try to speak at least a bit of the language whenever I visit a country.  However, I have been to some European countries where, at least in the main tourist areas, English seems to be almost universally understood and spoken.  It can be a bit disconcerting when you try to speak to someone in their native language, only to have them immediately switch into flawless English to reply (assuming of course that they understood what you tried to say in first instance).
I have also found that some countries don't seem to take kindly to their own language being mangled by a tourist, while others give you full marks for at least trying.
Can anyone tell me how widely English is spoken in Germany?  I would also appreciate thoughts on whether in a busy tourist area it is best to assume that English is spoken and start with that or to launch straight into German.
From my point of view, half the fun in travelling is to try to speak the language.  However, if the person I am talking to is flat out trying to take orders in a busy cafe, you don't want to make things more difficult for them if they can in fact speak English.
The other thing I want to ask is whether I can assume that all the phrases in Rocket German are OK to use in general polite conversation anywhere in Germany?  For example, in Premium Level 1 we learned the phrase "so ein Mist".  Where I live we would not normally say the equivalent when talking to people we don't know very well.  I once got into trouble with a Dutch family from the north of Holland by using a phrase which I had been told was perfectly acceptable in the south but was regarded quite differently in the north.

English is widely spoken in Germany, they learn English in school and large numbers of american and british troops have been stationed in Germany over the years.
I think they would appreciate you trying to speak German even if you end up in English if they speak it better.

As for colloquial expressions it's probably better to steer clear of them, it is hard to know when it is appropriate to use them and I think some are regional. I have a german friend who hadn't heard of half of the idiomatic expression given later in the course.

Many thanks for the feedback and the encouragement to at least start in German even if I then end up in English.  Although I am only half way through Premium 1, I was starting to lose confidence in ever being able to speak reasonably fluent German in Germany.
You have also confirmed what I was thinking about the colloquial expressions. 
Incidentally, I noticed in one of your other posts that you managed to insert umlauts in your post.  How do you do that?  Do I need to change my main keyboard in Windows?

I would be really nice if Rocket gave us a keyboard in the forum as they do in the write it tests. maybe we should suggest it?

I am trying to remember how I did the umlauts, I think it was with the character map app you get with windows. You can copy and paste accented letters from there. Or I might have copied & pasted a word from somewhere else like a dictionary.

Depends on location.  Across the country younger people and people in the service industry will have some degree of English.  However, in the former GDR (East Germany) English is a lot less common with anyone that came to age post 1989 being less likely to speak any English.  In towns like Wernigerode or Stralsund (two of my favorite places), you may not hear a single person that can speak English during a visit.  But in larger cities, like Berlin, English will be far more common.  

As to typing umlauts, you can set your computer to German, then just hold down the letter you need an umlaut for, like "U", and it will pop up with a selection to choose "ü".

Many thanks for the suggestion, JonathanB14, but I am using Windows 10 and it doesn't seem to work there.  Instead you have to remember which key will produce the letter you want with an umlaut.  Thanks also for the comments on where English is likely to be spoken.

Hi there,

@Graeme-TE1q: Windows 10 allows you to add multiple keyboards to the OS: click on "ENG" in the bottom right corner of your screen (the tag changes depending on the language you are currently using, "DEU", "ENG", etc.), open the language preferences and add the German keyboard. Once added, you can quickly skim through the added languages by pressing together the Windows button and the spacebar on your physical keyboard (the Windows button is in the bottom left corner). I'm on Win10 and currently have four languages enabled and they all work perfectly.