I know the conventional wisdom is to remove the winch handle when it's not in active use, but I'm a racer and fairly frequent singlehander, and do what is safe and efficient.
I usually leave the winch handle in the winch at all times, except in strong, gusty air when I might have to blow the sheet suddenly. I make small adjustments to sheet tension frequently, and it's inefficient to keep inserting and removing the winch handle every time, especially when singlehanding. Before each tack I put 2-3 wraps on the lazy winch and move the winch handle from the working winch to the lazy winch. By doing those things before the tack, I don't have to do them afterward, when time is limited and I don't have enough hands to do everything. The winch handle usually stays in the winch until the next tack.
, if winches are cleaned and lubed, the only way a winch handle would spin wildly would be if all the pawls broke at the same time. If that happened, I'm pretty sure I'd be smart enough to not try to grab it until it stopped spinning.
The winch handle can make it awkward if you need to put an extra wrap on the winch under load, but it can be done if you keep the line taut while you put on the extra wrap. Also, it might be in the way if you have a line override itself, but one handed winch handles can be removed easily, with one hand.
Generally, I think a winch handle should be in a winch if you intend to use it, and in a winch pocket otherwise, but, when I'm racing, it's used frequently.