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 $11.99   12.391%   $13.4757   $13.48   $13.50   $13.50  
  Purchase #2 $214.29   13.247%   $242.677   $242.68   $242.70   $242.70  
  Purchase #3 $215.75   5.355%   $227.3034   $227.30   $227.30   $227.30  
  Purchase #4 $6.40   13.250%   $7.248   $7.25   $7.25   $7.20  
  Purchase #5 $52.38   6.614%   $55.8444   $55.84   $55.85   $55.80  
  Purchase #6 $136.45   8.300%   $147.7754   $147.78   $147.80   $147.80  
  Purchase #7 $206.70   4.240%   $215.4641   $215.46   $215.45   $215.50  
  7 purchase totals = $843.96       $909.7880   $909.79   $909.85   $909.80