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 $29.77   9.199%   $32.5085   $32.51   $32.50   $32.50  
  Purchase #2 $180.50   8.233%   $195.3606   $195.36   $195.35   $195.40  
  Purchase #3 $199.85   5.271%   $210.3841   $210.38   $210.40   $210.40  
  Purchase #4 $50.62   9.109%   $55.231   $55.23   $55.25   $55.20  
  Purchase #5 $66.31   13.729%   $75.4137   $75.41   $75.40   $75.40  
  Purchase #6 $236.31   12.744%   $266.4253   $266.43   $266.45   $266.40  
  Purchase #7 $202.86   10.426%   $224.0102   $224.01   $224.00   $224.00  
  7 purchase totals = $966.22       $1,059.3334   $1,059.33   $1,059.35   $1,059.30