Yes, definitely leave the impeller out of the pump.
Case in point - just got my 4.4m side console fishing boat going again after ~18months with practically no use (coincidentally I bought Wine Down ~18months ago as well... do the maths
) The 40hp 4strk yammi had been fully serviced 12months ago as per it's usual yearly service which included a new impeller. So 12months since new impeller and NO use whatsoever. On servicing it last week - the unused impeller (basically a teenier version of the impellers in my Johnson pumps) was seen to have definite creases in several of the veins which my mechanic was adamant would cause failure at some point soon. I totally agreed with his assessment and a new impeller was sub $20 so it was no decision whatsoever to replace.
So long story short - yes, keep the impellers out until needed. And use glycerol as the lubricant (at least, that's what the volvo replacement impeller kit includes).