Some years ago I read a footnote warning in a blockheater manufacturer's catalog. OF course I don't recall the name of the outfit but the warning basically said it was recommended not to start the engine with the block heater plugged in because in certain cases the water flow can cause sufficient turbulence around the block heater element and result in a hot spot and shorten the life of the element.
IT makes good technical/theoretical sense to me. It seems less likely to be a problem at idle when coolant flow rates are low and turbulence is less than compared to roaring the engine upon starting like some people do. Personally I unplug mine.
Now I am also guessing/surmising on issue here. It seems likely that all replacement units are not exactly the same size and shape element as the original and that could be the root cause. Narrow water passages on some engines may make them more susceptible to the issue too. ANyhow, given the lack of a problem by many people that regularly use their block heaters in the winter and regularly start their engines with them still plugged in it seems safe to say it shouldn't be an issue with the original heater in the hemi. Not sure how many Pentastar users there are so it may not be fair to conclude the same for it.
All the best,,
And the above was written before I saw the note that the block heater on the gas engines doesn't go into the water jacket on the gas engines. Is that the Hemi and the Pentastar? Never heard of such a thing from the factory, but I guess it saves a place for a water jacket leak.