What I have found (the hard way) is that if you are not very aware during check out than all of your hard work may be for nothing. Often a cashier can look like the coupons are being used when they may not. I have found that in particular when you are using a large number of coupons that they may stick together or a cashier may skip some. Also, if a coupon beeps then the cashier will have to manually enter or override it and often they don't.
As a final check after I get home I go through and mark where I got each coupon taken off my receipt as well as to double check that I got the correct prices for sale items. I also keep and file every receipt so that if I find a rebate I am able to submit it for additional savings!
It is easy to get distracted when shopping with a child or when someone strikes up a conversation during your checkout. I recently had an experience where the lady checking out behind me started a conversation and I was not paying attention to the cashier. As a result I ended up not saving $18.00 from the sale. Had I immediately gone to the customer service desk I would have been able to get my savings back however I was in a rush to pick up my son so I decided to deal with it later. I had $14.00 in Meijer Mealbox Coupons not come off my total and $4.00 in Manufacturer Coupons. This made a HUGE difference on my savings. I ended up taking my items and receipts back and got $14.00 in cash back for the Meijer Mealbox Coupons because they scanned so I had proof. As for the $4.00 in manufacturer coupons there was no way to prove that they had been used so I lost that $4.00 in savings.
I learned the hard way to pay attention during checkout!