Torque estimating and the three-way hex wrench. 

Torque is the twisting level of force needed to turn something. Torque is measured in Nm (newton metres). Except in the case of carbon fibre or light weight metals, most handwork on bicycles is done without torque wrenches (which measure torque). It is done by experienced people and they, whether someone in a shop or an experienced mechanic at home, are intuitively judging tightness. In the workshops, "multi-tools" are not used at all. The typical allen key (bottom right picture) is also not used much. The mecahanics all use either three-way allen keys or special T-shaped tools (middle picture). 

We encourage you to acquire, if you do not already own one, a Y-hex wrench or "three way allen key". Get one when you go to the shop to learn how to retighten your A-headset. Learn to judge tightness as a comfortable wrist-twisting action. Don't ask to be taught with a multi-tool when the professionals use, for the major sizes of 4, 5 and 6mm, a three-way wrench or the T shaped wrench.

Taking the example of the handlebar stem screws, if they are too loose you might lose steering control and crash. If they are too tight they might break and will not be easy to replace.