The Church at this time gains most of it's power on loan from those who follow it. The King and Earl Rodrick are not particularly devout, so the Bishop of Salisbury has a position similar to a counsellor or noble in the Earl's service. The fact that the Earl did not charge you with this task means you should be free to refuse it.
Should the Church become elevated in prominence in some way, there may be future repercussions. But as things stand now, you should have no mundane fears about refusing the Church. The spiritual concerns I leave to you to interpret.
Having said that, doing this would earn you favour with the bishop, and while you have little to fear from him, having someone in his position singing your praises certainly would not hurt your standing with the Earl.