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 $39.75   6.188%   $42.2097   $42.21   $42.20   $42.20  
  Purchase #2 $77.71   12.142%   $87.1455   $87.15   $87.15   $87.10  
  Purchase #3 $70.69   9.807%   $77.6226   $77.62   $77.60   $77.60  
  Purchase #4 $129.75   9.760%   $142.4136   $142.41   $142.40   $142.40  
  Purchase #5 $56.87   3.329%   $58.7632   $58.76   $58.75   $58.80  
  Purchase #6 $159.60   13.274%   $180.7853   $180.79   $180.80   $180.80  
  Purchase #7 $82.50   11.226%   $91.7615   $91.76   $91.75   $91.80  
  7 purchase totals = $616.87       $680.7014   $680.70   $680.65   $680.70