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 $61.56   6.721%   $65.6974   $65.70   $65.70   $65.70  
  Purchase #2 $249.58   3.216%   $257.6065   $257.61   $257.60   $257.60  
  Purchase #3 $116.85   11.711%   $130.5343   $130.53   $130.55   $130.50  
  Purchase #4 $76.26   10.376%   $84.1727   $84.17   $84.15   $84.20  
  Purchase #5 $222.87   6.203%   $236.6946   $236.69   $236.70   $236.70  
  Purchase #6 $70.29   7.461%   $75.5343   $75.53   $75.55   $75.50  
  Purchase #7 $37.59   3.644%   $38.9598   $38.96   $38.95   $39.00  
  7 purchase totals = $835.00       $889.1996   $889.19   $889.20   $889.20