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 $27.99   12.784%   $31.5682   $31.57   $31.55   $31.60  
  Purchase #2 $16.71   6.711%   $17.8314   $17.83   $17.85   $17.80  
  Purchase #3 $5.36   3.264%   $5.535   $5.54   $5.55   $5.50  
  Purchase #4 $118.54   13.798%   $134.8961   $134.90   $134.90   $134.90  
  Purchase #5 $137.76   4.264%   $143.6341   $143.63   $143.65   $143.60  
  Purchase #6 $82.35   7.607%   $88.6144   $88.61   $88.60   $88.60  
  Purchase #7 $24.81   11.154%   $27.5773   $27.58   $27.60   $27.60  
  7 purchase totals = $413.52       $449.6565   $449.66   $449.70   $449.60