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 $75.53   5.137%   $79.41   $79.41   $79.40   $79.40  
  Purchase #2 $10.89   4.684%   $11.4001   $11.40   $11.40   $11.40  
  Purchase #3 $99.69   12.776%   $112.4264   $112.43   $112.45   $112.40  
  Purchase #4 $121.44   5.777%   $128.4556   $128.46   $128.45   $128.50  
  Purchase #5 $164.52   9.740%   $180.5442   $180.54   $180.55   $180.50  
  Purchase #6 $93.95   3.273%   $97.025   $97.03   $97.05   $97.00  
  Purchase #7 $27.98   7.583%   $30.1017   $30.10   $30.10   $30.10  
  7 purchase totals = $594.00       $639.3630   $639.37   $639.40   $639.30