Depends on the base figure, how much extra weight his legs will be supporting (through equipment, sculpted extras etc.) and where he'll be displayed - i.e. is he just going to hit the front of a display case if the shelf-gremlins give him a shove? Similarly, any figures near the custom that might tumble and take him with them? Or might you knock it reaching for another?
Personally I always use stands (the small, Kaiser brand things, far better than Sideshow's) - I feel safer and it helps take the weight off a figure's ankles. The figure might be standing nice and sturdy but an ankle failure, a knocked shelf, a 1/6th suicide attempt...
For me that little bit of metal around the waist (and Kaiser stands slip under jackets etc. nice and easily) is worth the risk of lost accessories, breakages, gouged armour etc.
Nick