I think that Liverpool as insurance is okay, but its about how confident you feel. If you know you definitely wont hit Sheffield dont bother with it. If you think that you might f*ck up so badly that you'll miss your firm by more than one grade then id insure a lower one. It depends how capable you feel you are, how driven you are to achieve it and how well you know yourself. If you know that when you do badly you really f*ck everything up go for a lower entrance like BBB/BBC maybe even lower. If you tend to only mess things up then you should be more than safe with Liverpool's ABB.
My offers were AA, AA, Unconditional/280 points(which i already have). So having an insurance wasnt a hard decision as if i missed my firm AA and my insurance was AA ive missed that too. So mine is the Unconditional. Although I could theoretically f*ck up that badly!