Dog One says
The possible risk to waiting for the ice to build up is that ice will form over the boot above its inflation height and there fore can't be removed.
This is now an obsolete theory.
From the FAA
"Previously, pilots delayed deploying the boots in order to avoid ice bridging. Ice bridging, ice formations above the furthest extension of the boots, occurred in older generation boots which were not powerful enough to completely shed ice. The commonly held belief was that premature use of even modern-day deicing boots, would cause ice formation beyond the system's capability to shed.
Research since the mid 1990s found that modern deicing boots do an effective job in both shedding ice and preventing ice bridging completely."
And from the Canadians (who know a thing or two about ice)
"Several generations of pilots operating aeroplanes with pneumatic de-icing boots have been cautioned against the dangers of ice bridging. Pilots were -- and are -- advised against activation of the pneumatic de-icing boots before sufficient ice has built up on the leading edge -- generally between ¼ and 1 inch -- out of concern that the ice would form the shape of the inflated boot, resulting in the boot inflating and deflating under a shell of ice, making de-icing impossible. Despite the widespread belief in this phenomenon within the pilot community and its coverage in numerous technical publications, its existence cannot be substantiated, either technically or anecdotally".