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 $28.35   5.810%   $29.9971   $30.00   $30.00   $30.00  
  Purchase #2 $74.92   13.430%   $84.9818   $84.98   $85.00   $85.00  
  Purchase #3 $63.28   9.368%   $69.2081   $69.21   $69.20   $69.20  
  Purchase #4 $160.48   3.505%   $166.1048   $166.10   $166.10   $166.10  
  Purchase #5 $121.34   8.837%   $132.0628   $132.06   $132.05   $132.10  
  Purchase #6 $228.37   4.764%   $239.2495   $239.25   $239.25   $239.20  
  Purchase #7 $202.77   7.481%   $217.9392   $217.94   $217.95   $217.90  
  7 purchase totals = $879.51       $939.5433   $939.54   $939.55   $939.50