As a general rule of thumb you want price to hold important reference levels on a closing basis. This means that you can allow for an intraday move below the mark. This is commonly referred to as a shakeout. However you must be aware that some shakeouts can be severe, especially in the high octane swing stocks. Due to this it totally depends on the individual situation if a trader should scale-out some quickly on the first intraday violation of support or wait until end of day with the risk to fall victim to a nasty shakeout. If a stock closes below the expected “support” w/o a shakeout it tells you that:
A) The stock changed character and could be in for a deeper pullback/consolidation or maybe it is gone for good
B) You failed at identifying a proper support level
C) The stock has only strong hands on board which are not prone to overreacting.
If the latter is the case you can scale-out and then simply scale-in again once price moves up through the mark tomorrow and or the days ahead. Large cap stocks which are widely followed move more like a ocean tanker so support and changing direction can be a multiday process which often sees closes below support.
The quintessence is this: You want your stock to show you that it adheres to proper support in one way or the other. If a stock stops doing what you want it to do you have to think about letting it go.