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 $32.73   8.210%   $35.4171   $35.42   $35.40   $35.40  
  Purchase #2 $68.74   8.183%   $74.365   $74.37   $74.35   $74.40  
  Purchase #3 $108.48   9.681%   $118.9819   $118.98   $119.00   $119.00  
  Purchase #4 $62.53   13.472%   $70.954   $70.95   $70.95   $71.00  
  Purchase #5 $217.95   5.482%   $229.898   $229.90   $229.90   $229.90  
  Purchase #6 $232.34   7.595%   $249.9862   $249.99   $250.00   $250.00  
  Purchase #7 $185.78   4.485%   $194.1122   $194.11   $194.10   $194.10  
  7 purchase totals = $908.55       $973.7144   $973.72   $973.70   $973.80