Is it really necessary to properly heat oil in a pan before frying veggies? As an example, we often lightly fry onions in (preferably) Olive Oil before adding them to any number of dishes, such as scrambled eggs for breakfast.

My talented cooking friends frequently remind me, "Don't put your veggies into the frying pan until the oil is hot!" The thing is, I personally can't tell the difference if I put my veggies in immediately after adding oil to the pan, or if I wait for the oil to properly heat up first. My friends also tell me, "Don't turn the heat up all the way either!"


Why do people recommend heating the oil first? Are there really benefits to waiting the 3-5 minutes?
If the answer to above is yes, how high should you turn the heat up to get the oil? Can I just set it to 100% for 1 minute to cook the oil faster? Yes I realize I have to watch the oil carefully otherwise chaos will ensue.
If you are looking to fry veggies then yes, it is necessary to pre-heat.  If you put them in a cold pan with oil, you start 'sweating' the veggies instead of frying them.

For example: if you put a sliced onion into a pan with hot oil, it will cook and have a nice golden brown color as it caramelizes.  Put that same sliced onion in a cold pan with cold oil and then add heat, and the onion will first turn translucent and lose moisture.

Both of these cooking methods have their uses, but they are different.
How do you know when the oil is ready to cook? It will start to shimmer a little before it reaches the smoking point. Put the veggies in when you see this shimmer, or at the very first wisp of smoke.