How to discourage tell-tales?

[cupcakes]

Am pondering this after seeing it mentioned in another thread. My ds is almost 5 and is a bit of a tell tale. It's only recently I've really noticed but I think he's been like this for a while. It used to be just in the context of fights between him and dd (2). "She hit me" etc and I think came about because he felt he was getting the short end of the stick - her crying is a lot more noticable and ds keeps being ticked off for certain crimes towards her. So I can see how it came about but it is getting embarrassing now because he tell tales on his friends pretty harmless behaviour! For example, we were at a friends house for a playdate (he also has a younger sister the same age as my dd) and when my dd started crying my ds ran through saying " hit her!". We were only in the next room and I am 99% sure that it was not a malicious act, just rough play and totally accidental. Understandably it really pee'd off ds's friend who then got really annoyed with him.
Ds is really quite bothered about being 'good' and gets very upset at the thought of being on the 'black cloud' at school (as opposed to the happy sun) but I think he's carrying it a bit far.
How can I teach ds when is the right time for telling me about things which are important (like bullying) and when to just leave it?

[Mud]

his peers will teach him soon enough I think

I tend to say 'oh dear' and continue to ignore them, but if its something that needs intervention will also say 'well how do you think you can get xx to share / do y' and make them go and do it

don't worry it'll sort itself out

[cupcakes]

Recently I have taken to telling ds and dd to "just sort it out between the two of you!" (usuakky when I'm on mumsnet)

[Mud]

that should do it too