Garage that replaced mine never bothered with spring compressors and safety, they did it exactly like you did, same as so many other people here
It's unlikely that ball joints would simply pop but as Starbuck explained better safe than sorry I suppose, even if its a one in a million chance, hence for safety reasons I'd either strap them temporarily or fit a standard spring compressor, you wouldn't be able to compress the spring with them but at least they would keep the spring in its current compressed state.
1997, 4.0 LWB X300 - Unlucky car but still a daily driver.
1988, 3.6S XJS - Never ending restoration project.
1994, 3.2S X300 - Accident damaged, October 2015. Gone!
1994, 3.2S XJ40 - Long gone.