Why do participants prefer to give input values that are "round numbers"?
The effect you are referring to is called heaping. Unfortunately, I am not aware of a source discussing the psychological aspect of this phenomenon, but I found an article about a statistical approach to heaped data.


  Roberts JM, Brewer DD. (2001) Measures and tests of heaping in
  discrete quantitative distributions. Journal of Applied Statistics
  28(7), 887-896. http://www.tandfonline.com/doi/abs/10.1080/02664760120074960
  
  Abstract   Heaping is often found in discrete quantitative data based on subject responses to open-ended interview questions or
  observer assessments. Heaping occurs when subjects or observers prefer
  some set of numbers as responses (e.g. multiples of 5) simply because
  of the features of this set. Although heaping represents a common type
  of measurement error, apparently no prior general measure of heaping
  exists. We present simple measures and tests of heaping in discrete
  quantitative data, illustrate them with data from an epidemiologic
  study, and evaluate the bias of these statistics. These techniques
  permit formal measurement of heaping and facilitate comparisons of the
  degree of heaping in data from different samples, substantive domains,
  and data collection methods.