I watched some of the show on tv recently and it inspired me to want to go there one year
I think earlier than later might be advisable as I get the feeling it might not be around much longer at least in its present form.
The show is run by the RHS and they have now set up shows at their major gardens ( Wisley,Rosemoor,Hyde Hall and Harlow Carr) and within the next couple of years they will be opening a new garden at Bridgewater which is only a few miles from Tatton so it would not surprise me if the Tatton show as it is now might move to Bridgewater for economic reasons. That is they will not have to hire the Tatton Park estate.
Hope I am wrong! Only time will tell!