What a hard question! This is such a personal question. Each of our situations and especially our finances are different. Only you can determine how you should proceed with this question.
Today, I purchased a few extra boxes of cold cereal for my three-month supply breakfast menu. The cereal wasn't on sale. I bought it anyway. I'm eager to complete my three-month supply and have the peace/confidence that comes from know that it's complete. On the other hand, when finances are tight, sales can make getting your storage affordable. You should not go into debt to get your storage! Nor should you buy things faster than you can afford them.
I have a budget for home storage. So, I usually look for sales. But, if I can't find a good price after a month or so, I might purchase my items anyway. I'll only buy a minimum amount. Or, if I have items to buy in a different category that are on sale, I may spend my food storage budget on those things and wait to buy others. Once you get a pretty good basic supply, then you have time to wait for the sales. You'll also figure out pretty quickly when and if certain items go on sale.