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 $25.79   7.839%   $27.8117   $27.81   $27.80   $27.80  
  Purchase #2 $195.83   10.780%   $216.9405   $216.94   $216.95   $216.90  
  Purchase #3 $19.44   4.872%   $20.3871   $20.39   $20.40   $20.40  
  Purchase #4 $138.48   5.719%   $146.3997   $146.40   $146.40   $146.40  
  Purchase #5 $96.97   12.566%   $109.1553   $109.16   $109.15   $109.20  
  Purchase #6 $225.55   5.158%   $237.1839   $237.18   $237.20   $237.20  
  Purchase #7 $22.86   9.578%   $25.0495   $25.05   $25.05   $25.00  
  7 purchase totals = $724.92       $782.9277   $782.93   $782.95   $782.90