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 $234.74   8.590%   $254.9042   $254.90   $254.90   $254.90  
  Purchase #2 $210.65   12.468%   $236.9138   $236.91   $236.90   $236.90  
  Purchase #3 $19.87   5.691%   $21.0008   $21.00   $21.00   $21.00  
  Purchase #4 $67.71   3.180%   $69.8632   $69.86   $69.85   $69.90  
  Purchase #5 $49.41   13.473%   $56.067   $56.07   $56.05   $56.10  
  Purchase #6 $24.91   10.566%   $27.542   $27.54   $27.55   $27.50  
  Purchase #7 $202.74   3.796%   $210.436   $210.44   $210.45   $210.40  
  7 purchase totals = $810.03       $876.7270   $876.72   $876.70   $876.70