Thanks, spoke to a good installer and he said as long as you get all the channels with a good picture/sound, there is no need to worry about signal strength. Still be interested to know what others get.
In terms of getting a good 'behaviour' strength isn't the only factor. The 'cleanliness' of the signal is hugely important, too. Low Bit-Error rates are the key to good behaviour, and you can get low rates from a weak, but very clean signal. Conversely you can get high error rates from strong but dirty signals.
For example, if your Digital signal suffers from the same problem that causes ghosting on analog, then, even if it's strong, you may still get errors.
It's Signal Quality that matters, Signal Strength doesn't.
Also the Signal Meters inside digiboxes vary and are not consistent, you need a satellite installer's meter thing to get an accurate reading.
After upgrading to Sky+ and had blocking problem -- the first installer had just changed the LNB and not the entire dish assembly -- I got the second installer to achieve 100% Signal Quality (measured on his meter) before I would sign. My SQ bars are solid grey.