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 $8.47   9.354%   $9.2623   $9.26   $9.25   $9.30  
  Purchase #2 $17.89   8.757%   $19.4566   $19.46   $19.45   $19.50  
  Purchase #3 $242.89   6.285%   $258.1556   $258.16   $258.15   $258.20  
  Purchase #4 $88.74   3.451%   $91.8024   $91.80   $91.80   $91.80  
  Purchase #5 $67.68   7.776%   $72.9428   $72.94   $72.95   $72.90  
  Purchase #6 $38.33   9.458%   $41.9553   $41.96   $41.95   $42.00  
  Purchase #7 $199.52   11.327%   $222.1196   $222.12   $222.10   $222.10  
  7 purchase totals = $663.52       $715.6946   $715.70   $715.65   $715.80