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 $12.75   5.588%   $13.4625   $13.46   $13.45   $13.50  
  Purchase #2 $59.46   10.192%   $65.5202   $65.52   $65.50   $65.50  
  Purchase #3 $50.94   9.668%   $55.8649   $55.86   $55.85   $55.90  
  Purchase #4 $52.75   8.141%   $57.0444   $57.04   $57.05   $57.00  
  Purchase #5 $180.91   4.508%   $189.0654   $189.07   $189.05   $189.10  
  Purchase #6 $113.34   13.822%   $129.0059   $129.01   $129.00   $129.00  
  Purchase #7 $116.66   11.398%   $129.9569   $129.96   $129.95   $130.00  
  7 purchase totals = $586.81       $639.9202   $639.92   $639.85   $640.00