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 $61.53   8.732%   $66.9028   $66.90   $66.90   $66.90  
  Purchase #2 $199.72   11.640%   $222.9674   $222.97   $222.95   $223.00  
  Purchase #3 $121.29   7.757%   $130.6985   $130.70   $130.70   $130.70  
  Purchase #4 $143.26   11.277%   $159.4154   $159.42   $159.40   $159.40  
  Purchase #5 $158.30   6.674%   $168.8649   $168.86   $168.85   $168.90  
  Purchase #6 $20.95   7.541%   $22.5298   $22.53   $22.55   $22.50  
  Purchase #7 $209.73   3.488%   $217.0454   $217.05   $217.05   $217.00  
  7 purchase totals = $914.78       $988.4242   $988.43   $988.40   $988.40