If you have a widescreen TV, you will only see the correct looking pictures if you set the digital decoder box to widescreen mode.
If you select to BBC NEWS 24, you can use this channel to check your picture. This channel always broadcasts in widescreen, and you can compare the location of the clock in the bottom-left and the press red in the top-left to these images.
J Lodge: Not really! unless you are in a weaker signal area and also intend to use an aerialamplifier in the vicinity, then you "may" have a problem now and again.
That said though, the main reason that its not always desirable to do as you intend is because there is always the danger of picking up mains borne interference, albeit this being very unlikely unless the aforementioned reception conditions applies.
In other words, "if" you normally receive a good signal then I would go ahead!