Do you try to squeeze out every bit of shampoo or toothpaste out of the bottle to get the most for your money? The Simple Dollar crunches some numbers to show that might not be worth your while: if you aren't sure, follow the "two second rule".
Basically, he suggests you divide the cost of the bottle by the number of uses you'd get out of it. That's your "per use value".
It's not an exact calculation but you should be able to estimate how many uses you get out of a bottle of shampoo or a tube of toothpaste. Once the easy uses are done and you have to start putting in time and effort to squeeze out the remaining bits, figure out of the if the per use value is worth that time and effort for you, or if you're better off just chucking it. If the last five uses are going to take a lot of effort and only add up to 20 cents worth of savings, you're just wasting your time.
If you want a simple rule to follow, if it's really easy and takes only a second or two, a little extra effort to get the last bit out is worth it; if it's not that easy, it better save multiple uses or it's not worth it.
I'd suggest reading the full post linked below to get a clearer idea and detailed number-crunching. He also explains how the one minute needed to fold up a tube might be worth it, but the same effort for a shampoo might not.
The Last Bit In The Container [The Simple Dollar]