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 $12.29   13.243%   $13.9176   $13.92   $13.90   $13.90  
  Purchase #2 $211.52   6.281%   $224.8056   $224.81   $224.80   $224.80  
  Purchase #3 $81.48   5.332%   $85.8245   $85.82   $85.80   $85.80  
  Purchase #4 $76.67   3.857%   $79.6272   $79.63   $79.65   $79.60  
  Purchase #5 $74.33   12.797%   $83.842   $83.84   $83.85   $83.80  
  Purchase #6 $20.64   12.455%   $23.2107   $23.21   $23.20   $23.20  
  Purchase #7 $7.98   7.811%   $8.6033   $8.60   $8.60   $8.60  
  7 purchase totals = $484.91       $519.8309   $519.83   $519.80   $519.70