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 $244.90   11.584%   $273.2692   $273.27   $273.25   $273.30  
  Purchase #2 $57.39   6.199%   $60.9476   $60.95   $60.95   $60.90  
  Purchase #3 $173.56   7.405%   $186.4121   $186.41   $186.40   $186.40  
  Purchase #4 $97.49   7.322%   $104.6282   $104.63   $104.65   $104.60  
  Purchase #5 $62.55   10.428%   $69.0727   $69.07   $69.05   $69.10  
  Purchase #6 $171.26   7.695%   $184.4385   $184.44   $184.45   $184.40  
  Purchase #7 $6.76   11.137%   $7.5129   $7.51   $7.50   $7.50  
  7 purchase totals = $813.91       $886.2812   $886.28   $886.25   $886.20