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 $39.31   3.729%   $40.7759   $40.78   $40.80   $40.80  
  Purchase #2 $186.97   3.512%   $193.5364   $193.54   $193.55   $193.50  
  Purchase #3 $176.42   6.721%   $188.2772   $188.28   $188.30   $188.30  
  Purchase #4 $114.49   4.347%   $119.4669   $119.47   $119.45   $119.50  
  Purchase #5 $167.53   7.570%   $180.212   $180.21   $180.20   $180.20  
  Purchase #6 $195.81   11.131%   $217.6056   $217.61   $217.60   $217.60  
  Purchase #7 $215.69   10.336%   $237.9837   $237.98   $238.00   $238.00  
  7 purchase totals = $1,096.22       $1,177.8577   $1,177.87   $1,177.90   $1,177.90