depends on the situation, i got called a grass at work a while back as some old twat thought it was funny to constantly fuck up your work meaning you had to start again or bin what you were doing but i'd had enough one day, it was a choice between laying him out or talking to my boss, i didn't see the point lossing my job over the kind of person he was so did the latter. to this day he calls me all the names under the sun but everyone knows what a cunt he is and he ended up back on the shitty job he was on before so in that situation i feel i was right to "grass" as it saved me potentially lossing my job.