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You have to understand that flash photography ALWAYS involves two exposures. One exposure is made by the ambient light and the other by the flash.
Problems arise when the exposure of the two is close, like within three or four stops. When this happens, you can see a noticeable double image.
Normally, when using a flash, it is the dominant light source and the ambient exposure is 10-12 stops off, so it does not enter into the equation.
As others have pointed out, the flash speed is likely no slower than 1/1000 second. Many flashes are much faster, so the flash can stop most any movement.
If the ambient light is bright enough that 1/180 second at whatever f-stop and ISO setting you're using will form a recognizeable image (albeit probably underexposed), you have a problem.
In a studio setting, you usually have complete control of the ambient light, so you should be able to use a non-HSS flash with no problems.
HSS is handy when you are in a situation where you can't control the ambient light, such as outside. If you are using the flash as fill lighting to brighten faces in a brightly lit situation, then the ambient becomes important. In that case, using HSS lets you set the shutter speed high enough that the ambient exposure won't register. In a studio, this shouldn't be necessary.
BTW, high-speed-sync does not do a very good job of stopping motion, unless you can set the shutter speed high enough that it can stop the motion. Someone, somewhere posted pictures of a desk fan, one taken with normal flash and one taken with HSS flash. The normal flash froze the fan blades perfectly, while the HSS image showed the fan blades quite blurred. This is because, the P-TTL flash duration was on the order of 1/10,000 second, while the HSS shutter speed was slower than 1/1000.
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Post #5 by pschlute
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