Shopping for the whole month or week-by-week?

Ok so we have been shopping for the whole month lately...i would say the passed 2 or 3 months since we have had our own place...I am wondering...Do you seem to save more money by shopping once for the whole month or by shopping week-by-week? We usually spend anywhere between $300-$350 in groceries when we shop for the whole month...I have a family of 3 plus a baby that eats baby food and a step son on the weekends...so technically i guess that is 4 1/2...LOL...We are trying to do everything we can to not spend so much money on things. This includes groceries, gas, bills, etc...If you have any tips or ideas please drop a comment and let me know! Thanks!