safe i have a few a diff weights betwwen 90g - 140g, depends what you want, the heavier they are the slower they will spin but the more momentum they will have when there faster, also better for iso's. i fill balloons with rice or sand, i use a funnel and chopstick plus once you have it at your required weight suck the air out of the balloon then tie it, i feel this gives you a nicer ball (not necessary its my personal preference really. then i also but that balloon into another balloon so i can just stick the poi in the wash without taking the balls out (easier when you have weighted handles)
hope that helps
