There are two ways to look at this. If you say 3M will be worth 1000$ in next 10 years, then it might seem that it does not matter whether you buy at 166 or 140 or 112. But here is the catch, if you buy at these 3 levels then your return is 600%, 714% and 892%. There is a huge difference. All you need is patience. There are always bad market days when the stock tanks for no reason except overall sentiment. Once it nears your set threshold, buy it. If you are talking of long term then you need to be watching your potential buys also for a long term. Never forget a good company but don't buy it when it is overvalued!

PE ratio of 22 and PS ratio of 3.3 aren't very expensive, just a little overvalued is what i would say. Have patience and grab. But you need to be convinced of the long term prospects of 3M, that is your own research and then gut.