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 $19.90   6.374%   $21.1684   $21.17   $21.15   $21.20  
  Purchase #2 $91.70   9.617%   $100.5188   $100.52   $100.50   $100.50  
  Purchase #3 $167.34   11.275%   $186.2076   $186.21   $186.20   $186.20  
  Purchase #4 $82.57   12.805%   $93.1431   $93.14   $93.15   $93.10  
  Purchase #5 $60.26   5.338%   $63.4767   $63.48   $63.50   $63.50  
  Purchase #6 $236.75   13.437%   $268.5621   $268.56   $268.55   $268.60  
  Purchase #7 $164.37   7.815%   $177.2155   $177.22   $177.20   $177.20  
  7 purchase totals = $822.89       $910.2922   $910.30   $910.25   $910.30