It is not a hard and fast rule but in most companies the wait for 'right to left' on a LH Jet can be a long one. Hence many pilots, especially those who joined the airline with no previous command experience, go from right to left into SH and then bid back to RHS LH at every opportunity. There is no requirement to do so though.
Obviously you can only have such a rule if your airline offers the ability to do so. Virgin, for example, could have a few snags with such a ruling!