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 $19.28   3.565%   $19.9673   $19.97   $19.95   $20.00  
  Purchase #2 $123.33   6.244%   $131.0307   $131.03   $131.05   $131.00  
  Purchase #3 $142.81   7.192%   $153.0809   $153.08   $153.10   $153.10  
  Purchase #4 $215.48   10.267%   $237.6033   $237.60   $237.60   $237.60  
  Purchase #5 $64.92   8.256%   $70.2798   $70.28   $70.30   $70.30  
  Purchase #6 $108.71   11.557%   $121.2736   $121.27   $121.25   $121.30  
  Purchase #7 $175.61   12.175%   $196.9905   $196.99   $197.00   $197.00  
  7 purchase totals = $850.14       $930.2261   $930.22   $930.25   $930.30