Sorry I totally disagree with this, except for the speaker wire bit. For digital signals, especially for short runs, the signal will either be good or it wont. Its all sent as 1's and 0's, you cant get a half or anything stupid like that, so the signal is either good or not, no in between. As long as the wires are reasonable and shielded from interferance, you will notice no difference between a £5 cable and a £50 cable - but I agree a 99p cable might be shoddy and never give any signal. The signal is either intact or it is not. Now for longer runs or areas where you can get a lot of RFI that may not be the case but for 9 of 10 setups where you are running only 1-2m of cable you wont notice any difference. You will see a far greater difference having your TV profiled properly so that colours show up as their true colour
Speaker cables on the other hand are well worth spending a bit of cash on if you have it. As the signal is analogue, over much longer runs and open to more interferance you will notice the difference with better cables.