Unless you have the CP to spare for multiple Great Strides, you ALWAYS want it on the BEST Touch you can make. This means the last one, the one with the highest IQ stack - Byregot's if at all possible.

If, for a given recipe and stats, you get 100, 110, 121, 131, 142 for the first 5 Touches, why would you want to GS the 100 (bringing it to 200) when you could GS the fifth (bringing it to 284)?