Yeah, it's safe if keep it in eth form. Everyone should know the market has suffered throughout 2018, for the next few months there will definitely be a price improvement in movement. So, the advice you say is quite unique and it is indeed a possibility.
but I think don't gives much results if saved in ETH, if I would prefer to buy coins or tokens that are cheaper for example str, or neo. with 5 eth I think the growth profit is much bigger than if stored in ETH coin.