:AOK I've used these, old tennis balls and odd foam stress balls. My favorites the past few years have been a couple of stress ball brains; perfect density and shape for fiddling it between fingertips or gripping with all the tips (like a baseball knuckleball pitcher's grip)
Finger independence, true. One of the means to that end, and to nimble control of 'touch', is good finger muscle tone. It's not about developing brute strength. Squeezing too tight has a negative effect on tone and finger quickness. But strong, long finger muscles -- akin to a swimmer's or to a juice-free baseball pitcher's arm -- in both hands, make for better control and precision of movement. They allow you to relax, to minimize effort. And ironically, you find yourself playing with a lighter touch as a result.