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 $129.77   3.707%   $134.5806   $134.58   $134.60   $134.60  
  Purchase #2 $152.83   6.607%   $162.9275   $162.93   $162.95   $162.90  
  Purchase #3 $59.86   11.147%   $66.5326   $66.53   $66.55   $66.50  
  Purchase #4 $126.71   6.630%   $135.1109   $135.11   $135.10   $135.10  
  Purchase #5 $14.90   11.172%   $16.5646   $16.56   $16.55   $16.60  
  Purchase #6 $63.96   3.779%   $66.377   $66.38   $66.40   $66.40  
  Purchase #7 $62.48   11.292%   $69.5352   $69.54   $69.55   $69.50  
  7 purchase totals = $610.51       $651.6284   $651.63   $651.70   $651.60