The best time is usually before your main weekly shop, not while you are already halfway through adding random items to a basket. Price comparison works best when you use it as a planning step.
For many households, one main restock trip or delivery basket drives most weekly spending. That is the right moment to compare prices. If you wait until checkout, convenience takes over and you are less likely to switch stores or rebalance your basket.
Items like rice, milk, Milo, detergent, diapers, and tissue are excellent comparison triggers. You already know you will buy them again, so even a small price difference matters more than a flashy discount on something you rarely use.
A promotion looks attractive only when you know the normal range. Compare prices across merchants so you can tell whether a deal is genuinely strong or simply more visible.
If driving distance affects where you shop, checking weekly fuel prices alongside grocery planning can be useful. That is especially true when you are deciding whether one larger trip or a nearby store makes more sense.