Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $198.93   12.214%   $223.2273   $223.23   $223.25   $223.20  
  Purchase #2 $156.81   8.140%   $169.5743   $169.57   $169.55   $169.60  
  Purchase #3 $141.65   5.850%   $149.9365   $149.94   $149.95   $149.90  
  Purchase #4 $53.34   7.834%   $57.5187   $57.52   $57.50   $57.50  
  Purchase #5 $158.71   12.126%   $177.9552   $177.96   $177.95   $178.00  
  Purchase #6 $154.58   6.496%   $164.6215   $164.62   $164.60   $164.60  
  Purchase #7 $16.27   5.559%   $17.1744   $17.17   $17.15   $17.20  
  7 purchase totals = $880.29       $960.0079   $960.01   $959.95   $960.00