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 $103.65   4.423%   $108.2344   $108.23   $108.25   $108.20  
  Purchase #2 $126.53   9.190%   $138.1581   $138.16   $138.15   $138.20  
  Purchase #3 $72.27   4.623%   $75.611   $75.61   $75.60   $75.60  
  Purchase #4 $182.85   9.614%   $200.4292   $200.43   $200.45   $200.40  
  Purchase #5 $36.76   9.139%   $40.1195   $40.12   $40.10   $40.10  
  Purchase #6 $89.97   10.657%   $99.5581   $99.56   $99.55   $99.60  
  Purchase #7 $20.36   12.867%   $22.9797   $22.98   $23.00   $23.00  
  7 purchase totals = $632.39       $685.0900   $685.09   $685.10   $685.10