Frequency of behaviors or amounts of variables of interest are essential topics in many surveys. The use of heuristics might cause rounded answers, resulting in the increased occurrence of end-digits (called heaping or digit-preference). For web surveys (or CASI), we propose using a conditional prompt as input validation if digits indicating heaping are entered. We report an experiment, where respondents in an online access panel (n = 2,590) were randomly assigned to one of three groups: (1) no input validation; (2) conditional input validation if rounding was presumed; and (3) input validation every time a numerical value was entered. Conditional input validation reduces heaping for variables with high proportions of heaped values. Unconditional input validation seems to be less effective.