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 $125.26   6.651%   $133.591   $133.59   $133.60   $133.60  
  Purchase #2 $244.87   6.172%   $259.9834   $259.98   $260.00   $260.00  
  Purchase #3 $8.92   7.482%   $9.5874   $9.59   $9.60   $9.60  
  Purchase #4 $142.51   8.427%   $154.5193   $154.52   $154.50   $154.50  
  Purchase #5 $126.67   11.841%   $141.669   $141.67   $141.65   $141.70  
  Purchase #6 $218.93   11.549%   $244.2142   $244.21   $244.20   $244.20  
  Purchase #7 $42.84   10.204%   $47.2114   $47.21   $47.20   $47.20  
  7 purchase totals = $910.00       $990.7757   $990.77   $990.75   $990.80