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 $125.43   10.353%   $138.4158   $138.42   $138.40   $138.40  
  Purchase #2 $100.77   8.713%   $109.5501   $109.55   $109.55   $109.60  
  Purchase #3 $195.25   8.637%   $212.1137   $212.11   $212.10   $212.10  
  Purchase #4 $19.49   4.779%   $20.4214   $20.42   $20.40   $20.40  
  Purchase #5 $205.96   7.168%   $220.7232   $220.72   $220.70   $220.70  
  Purchase #6 $119.95   4.639%   $125.5145   $125.51   $125.50   $125.50  
  Purchase #7 $75.83   13.272%   $85.8942   $85.89   $85.90   $85.90  
  7 purchase totals = $842.68       $912.6329   $912.62   $912.55   $912.60