It depends how you want the bike to handle, how you like to ride. I found a huge thread on MTBR talking about fork offset, very interesting to read.
I'm used to ride my XC bike (69 HTA) with an old rockshox with 46mm offset, when i tested a bike with 51 offset and same HTA, i found more or less what is said on MTBR. I prefer my 46 offset