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 $138.72   3.540%   $143.6307   $143.63   $143.65   $143.60  
  Purchase #2 $89.27   5.845%   $94.4878   $94.49   $94.50   $94.50  
  Purchase #3 $230.28   8.165%   $249.0824   $249.08   $249.10   $249.10  
  Purchase #4 $28.89   13.580%   $32.8133   $32.81   $32.80   $32.80  
  Purchase #5 $51.82   8.760%   $56.3594   $56.36   $56.35   $56.40  
  Purchase #6 $33.92   7.841%   $36.5797   $36.58   $36.60   $36.60  
  Purchase #7 $50.74   4.429%   $52.9873   $52.99   $53.00   $53.00  
  7 purchase totals = $623.64       $665.9406   $665.94   $666.00   $666.00