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 $18.61   9.520%   $20.3817   $20.38   $20.40   $20.40  
  Purchase #2 $208.52   11.775%   $233.0732   $233.07   $233.05   $233.10  
  Purchase #3 $5.30   7.785%   $5.7126   $5.71   $5.70   $5.70  
  Purchase #4 $137.44   4.466%   $143.5781   $143.58   $143.60   $143.60  
  Purchase #5 $143.52   12.113%   $160.9046   $160.90   $160.90   $160.90  
  Purchase #6 $18.81   11.435%   $20.9609   $20.96   $20.95   $21.00  
  Purchase #7 $187.82   13.494%   $213.1644   $213.16   $213.15   $213.20  
  7 purchase totals = $720.02       $797.7755   $797.76   $797.75   $797.90