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 $81.48   3.468%   $84.3057   $84.31   $84.30   $84.30  
  Purchase #2 $53.76   4.501%   $56.1797   $56.18   $56.20   $56.20  
  Purchase #3 $94.39   11.571%   $105.3119   $105.31   $105.30   $105.30  
  Purchase #4 $75.45   9.226%   $82.411   $82.41   $82.40   $82.40  
  Purchase #5 $50.30   11.676%   $56.173   $56.17   $56.15   $56.20  
  Purchase #6 $217.79   13.267%   $246.6842   $246.68   $246.70   $246.70  
  Purchase #7 $60.74   8.681%   $66.0128   $66.01   $66.00   $66.00  
  7 purchase totals = $633.91       $697.0783   $697.07   $697.05   $697.10