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 $19.88   3.801%   $20.6356   $20.64   $20.65   $20.60  
  Purchase #2 $170.85   6.507%   $181.9672   $181.97   $181.95   $182.00  
  Purchase #3 $103.30   3.235%   $106.6418   $106.64   $106.65   $106.60  
  Purchase #4 $38.38   7.845%   $41.3909   $41.39   $41.40   $41.40  
  Purchase #5 $181.56   10.521%   $200.6619   $200.66   $200.65   $200.70  
  Purchase #6 $208.94   11.112%   $232.1574   $232.16   $232.15   $232.20  
  Purchase #7 $128.61   12.687%   $144.9268   $144.93   $144.95   $144.90  
  7 purchase totals = $851.52       $928.3816   $928.39   $928.40   $928.40