As Kchriste explains, the buck converter needs only to worry about the average current, in theory, not the peak current. The peak current is supposed to be supplied by bypass capacitors, but if you have peak demands that last a longer amount of time, like many milliseconds for example, then it is possible that bypass capacitors can't supply enough, in which case your regulator may have to supply those longer peaks as well. However, CMOS counters and flip-flops and the like have very fast switching times and very short transient current demands, so the peaks will be very short. Bypass capacitors should be up to handling these.