We buy as many groceries as we can in bulk but always look at the unit price to check that this is a cost saving. For instance the corn oil can above was 10 pounds for 15 litres of oil, a considerable saving on the cost of buying 10 separate litre bottles of oil. However, you do have to have somewhere to store all your bulk purchases and I have found that with the more popular items you may also have to hide your stockpile as otherwise the family will eat it all in half the time. This is very true of chocolate, where I am usually behind Master Shoestring's ability to work out where I have hidden the stuff !
We have found that there are great deals to be had on bulk items in supermarkets now as they start to get larger and give discount warehouses such as Costco a run for their money. We did take out a Costco membership a few years ago and I'm still using the bulk herbs and vanilla essence which I got there but for us an annual membership did not make sense as it is a long way away and their prices on meat and vegetables and fruit were the same as we could find locally.
We also have quite a few ethnic grocers nearby so that also helps with bulk buying at low cost. Knowing exactly when a good deal is an unbeatable one relies on knowing the regular unit price of things you buy on a regular basis. For that you need to keep some sort of price record, I used to keep a book with all the standard prices in it but now I can use my supermarket online to keep up to date with pricing across all our local stores and only need to track prices from elsewhere, such as the local grocers. This is well worth doing as it also let's you take advantage of good online deals or tells you when you shouldn't !
There is another disadvantage to stockpiling in addition to eating through it too fast and that is eating though it too slowly, 2 years ago I had to throw out a lot of parsnips when I bought 2o pounds and we didn't eat through them fast enough and they went mouldy, I was cross with myself as had I realise earlier I could have frozen them ! So stockpile with care and keep a record of where everything is !
Do you stockpile groceries ?