It's very unexciting for the viewer with no particular contestant preference, if the best goes first, and the producers know who is likely to be best from the rehearsal, and previous form. If they put Ray, Hayley and Beth on in the first half, the viewers may switch off. The TV company is in it for the revenue, they have no other particular bias.
I don't think running order is affecting the votes that much, the only person who got votes above his ability was Gareth, and he's been one of the early ones anyway. Sam was probably getting votes below his ability earlier in the series and somehow I don't think that's due to running order either.