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 $133.91   3.588%   $138.7147   $138.71   $138.70   $138.70  
  Purchase #2 $44.83   5.131%   $47.1302   $47.13   $47.15   $47.10  
  Purchase #3 $127.67   9.452%   $139.7374   $139.74   $139.75   $139.70  
  Purchase #4 $51.50   8.390%   $55.8209   $55.82   $55.80   $55.80  
  Purchase #5 $159.75   6.761%   $170.5507   $170.55   $170.55   $170.60  
  Purchase #6 $143.66   13.720%   $163.3702   $163.37   $163.35   $163.40  
  Purchase #7 $90.48   11.441%   $100.8318   $100.83   $100.85   $100.80  
  7 purchase totals = $751.80       $816.1559   $816.15   $816.15   $816.10