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 $231.51   12.635%   $260.7613   $260.76   $260.75   $260.80  
  Purchase #2 $66.70   4.145%   $69.4647   $69.46   $69.45   $69.50  
  Purchase #3 $168.82   8.174%   $182.6193   $182.62   $182.60   $182.60  
  Purchase #4 $59.81   8.607%   $64.9578   $64.96   $64.95   $65.00  
  Purchase #5 $172.60   9.137%   $188.3705   $188.37   $188.35   $188.40  
  Purchase #6 $154.94   10.767%   $171.6224   $171.62   $171.60   $171.60  
  Purchase #7 $78.26   7.227%   $83.9159   $83.92   $83.90   $83.90  
  7 purchase totals = $932.64       $1,021.7119   $1,021.71   $1,021.60   $1,021.80