If I remember correctly (and I infrequently do) signalling to pass parked cars can be misunderstood by other motorists as an intention to turn into a side ride, and therefore can be misleading.
If a signal can be ambiguous then don't signal. Signalling around an object is likely to cause an ambiguous signal as it can often become hard to tell the difference between someone pulling to the right ahead & someone simply going round something in the road.got it in one, but often there is nothing to turn into, but just on a common sense level, why signal to go around something we are all going to assume you are not going to drive through? (sorry I used to teach advanced level driving for +18 years or so, so I tend to be a bore on this matter!).... (and yep - common sense does not often apply to motorists, but driving through a parked car?)
@maz - it's a should rather than a must.
Couple of points:
When turning left, it is often safer not to signal because; (i) you have less control over the bike and (ii) it stops cars turning across you where they are waiting to turn right (unfortunately when signalling an intention, some road users will take chances and make risky manouvers).