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 $241.37   13.382%   $273.6701   $273.67   $273.65   $273.70  
  Purchase #2 $246.33   10.643%   $272.5469   $272.55   $272.55   $272.50  
  Purchase #3 $109.46   10.496%   $120.9489   $120.95   $120.95   $120.90  
  Purchase #4 $130.99   3.416%   $135.4646   $135.46   $135.45   $135.50  
  Purchase #5 $120.37   9.522%   $131.8316   $131.83   $131.85   $131.80  
  Purchase #6 $213.76   6.327%   $227.2846   $227.28   $227.30   $227.30  
  Purchase #7 $170.75   9.210%   $186.4761   $186.48   $186.50   $186.50  
  7 purchase totals = $1,233.03       $1,348.2228   $1,348.22   $1,348.25   $1,348.20