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 $57.71   13.425%   $65.4576   $65.46   $65.45   $65.50  
  Purchase #2 $66.41   12.796%   $74.9078   $74.91   $74.90   $74.90  
  Purchase #3 $134.29   5.673%   $141.9083   $141.91   $141.90   $141.90  
  Purchase #4 $223.56   4.259%   $233.0814   $233.08   $233.10   $233.10  
  Purchase #5 $72.25   9.708%   $79.264   $79.26   $79.25   $79.30  
  Purchase #6 $65.76   6.208%   $69.8424   $69.84   $69.85   $69.80  
  Purchase #7 $125.88   11.571%   $140.4456   $140.45   $140.45   $140.40  
  7 purchase totals = $745.86       $804.9071   $804.91   $804.90   $804.90