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 $220.76   12.419%   $248.1762   $248.18   $248.20   $248.20  
  Purchase #2 $97.74   8.648%   $106.1926   $106.19   $106.20   $106.20  
  Purchase #3 $13.89   9.169%   $15.1636   $15.16   $15.15   $15.20  
  Purchase #4 $229.98   10.843%   $254.9167   $254.92   $254.90   $254.90  
  Purchase #5 $80.66   4.633%   $84.397   $84.40   $84.40   $84.40  
  Purchase #6 $18.54   6.766%   $19.7944   $19.79   $19.80   $19.80  
  Purchase #7 $166.50   13.676%   $189.2705   $189.27   $189.25   $189.30  
  7 purchase totals = $828.07       $917.9110   $917.91   $917.90   $918.00