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 $19.37   10.159%   $21.3378   $21.34   $21.35   $21.30  
  Purchase #2 $118.52   8.531%   $128.6309   $128.63   $128.65   $128.60  
  Purchase #3 $170.93   9.532%   $187.223   $187.22   $187.20   $187.20  
  Purchase #4 $50.69   12.582%   $57.0678   $57.07   $57.05   $57.10  
  Purchase #5 $212.45   12.642%   $239.3079   $239.31   $239.30   $239.30  
  Purchase #6 $84.61   10.741%   $93.698   $93.70   $93.70   $93.70  
  Purchase #7 $234.96   6.153%   $249.4171   $249.42   $249.40   $249.40  
  7 purchase totals = $891.53       $976.6825   $976.69   $976.65   $976.60