In theory as generally suggested, new tires is better at the front for safety reason especially in the wet. This is also what I have said if anyone asked in forums coz in general I would not know how they drive and what are the condition of their older tires. So for the sake of safety and not wanting to be guilty to lead someone to having an accident, that's what I'd normally suggest too. However for me personally, it would depend on the old tire condition, how different they are to the newly bought one, or how bad they are already. Because in practice when I do front/back rotation every 10-20k km, of course I'd still end up putting the slightly better tires at the front, yet it's still working fine, no spins or as scary as had been portrayed by the theory. In past, I also had put new tires at the front too coz I like the fast responsive feel of new tires at the front. However I never had so big of a difference between new & old tires between the front and rear (my old rear were never reaching bad condition in the first place), and I do keep in mind the risk of it especially in the wet and therefore I drive accordingly. Anyway the best solution is to always try to rotate regularly every 10-20k km so the wear will be more even and replace all 4 and that's mostly what I did before (cannot remember when was the last time I just changed 2 new tires to be honest). Even with my current staggered setup, last time I replaced all 4 tires even though my rears still had like 2mm deeper treads. So if possible, rotate your tires hence the tire will last longer for all 4 but then must change all 4 together too, that is the best practice IMHO. If really you can only replace 2 then since I don't know how you drive or how good you are and how are your old tires, then I'd suggest to put them at the back for better safety reason. :)