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 $20.32   3.336%   $20.9979   $21.00   $21.00   $21.00  
  Purchase #2 $128.27   3.145%   $132.3041   $132.30   $132.30   $132.30  
  Purchase #3 $53.71   10.724%   $59.4699   $59.47   $59.45   $59.50  
  Purchase #4 $238.38   4.693%   $249.5672   $249.57   $249.55   $249.60  
  Purchase #5 $98.44   5.418%   $103.7735   $103.77   $103.75   $103.80  
  Purchase #6 $228.49   13.770%   $259.9531   $259.95   $259.95   $260.00  
  Purchase #7 $143.84   13.232%   $162.8729   $162.87   $162.85   $162.90  
  7 purchase totals = $911.45       $988.9386   $988.93   $988.85   $989.10