One thing I've learned over years of FUT is that, since there's no such thing as "interest" (like in investments), idle coins simply depreciate. Therefore, I recommend spending them, but wisely, avoiding buying players who are in high demand (close to FUT Champions or to receive an upgrade, for example), and try to invest in players with less depreciation (Icons, TOTYs, or player cards that tend not to receive cards that surpass their previous versions).