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 $75.73   3.669%   $78.5085   $78.51   $78.50   $78.50  
  Purchase #2 $164.93   8.181%   $178.4229   $178.42   $178.40   $178.40  
  Purchase #3 $96.53   10.470%   $106.6367   $106.64   $106.65   $106.60  
  Purchase #4 $65.35   10.566%   $72.2549   $72.25   $72.25   $72.30  
  Purchase #5 $207.67   11.528%   $231.6102   $231.61   $231.60   $231.60  
  Purchase #6 $226.31   11.727%   $252.8494   $252.85   $252.85   $252.80  
  Purchase #7 $58.56   12.757%   $66.0305   $66.03   $66.05   $66.00  
  7 purchase totals = $895.08       $986.3131   $986.31   $986.30   $986.20