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 $76.27   3.241%   $78.7419   $78.74   $78.75   $78.70  
  Purchase #2 $124.66   11.690%   $139.2328   $139.23   $139.25   $139.20  
  Purchase #3 $212.85   7.115%   $227.9943   $227.99   $228.00   $228.00  
  Purchase #4 $233.48   11.144%   $259.499   $259.50   $259.50   $259.50  
  Purchase #5 $195.39   7.534%   $210.1107   $210.11   $210.10   $210.10  
  Purchase #6 $19.83   12.796%   $22.3674   $22.37   $22.35   $22.40  
  Purchase #7 $242.36   13.577%   $275.2652   $275.27   $275.25   $275.30  
  7 purchase totals = $1,104.84       $1,213.2113   $1,213.21   $1,213.20   $1,213.20