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 $7.87   4.280%   $8.2068   $8.21   $8.20   $8.20  
  Purchase #2 $46.67   3.813%   $48.4495   $48.45   $48.45   $48.40  
  Purchase #3 $180.97   9.249%   $197.7079   $197.71   $197.70   $197.70  
  Purchase #4 $17.70   10.455%   $19.5505   $19.55   $19.55   $19.60  
  Purchase #5 $46.61   12.722%   $52.5397   $52.54   $52.55   $52.50  
  Purchase #6 $142.87   5.146%   $150.2221   $150.22   $150.20   $150.20  
  Purchase #7 $168.66   8.104%   $182.3282   $182.33   $182.35   $182.30  
  7 purchase totals = $611.35       $659.0047   $659.01   $659.00   $658.90