I don't like to pre-book the first time I go to a new parlour, as until you get through the doors you don't know for certain what the place will be like or what the WGs will be like.
It has backfired occasionally when the WG I am interested in was very busy and the not busy WGs on the same shift are not busy because they were bad. Sometimes though you go with a WG you weren't planning to see and have a great time.
But, I always block out a big section of time in a day for punting and have a plan B, C, D and E. If going to Roses I would familiarise myself with the Sheridans and Temptations rotas for the same day at minimum as the three are quite close together if you are in a car. And depending on what direction you are going into/out of Manchester you'll probably go past another parlour too.
Even if you don't see the WG you specifically want in the line up, you might see 2 or 3 others to get an idea on their actual looks as compared to reviews.