I don't personally think it's a good idea to chop and change too much as it could make them fussy.
What I did with Alfie was keep him on what the breeder gave me (Skinners) for the first 2-3 weeks, then changed him over slowly to Royal Canin (it was on half-price offer at my local pet shop). I also sent off for samples of several different foods, including Burns, The Natural Dog Food Company, CSJ, Barking Heads and Arden Grange, and used them as treats/training rewards. That way I got to try them out on Alfie, but obviously in much smaller quantities. I liked the Arden Grange sized kibble, I liked their ethics, the food smelled ok, and they did a variety of flavours. So then when Alfie was 5 months old I changed him over to Arden Grange over about 4-5 days.