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 $17.71   12.290%   $19.8866   $19.89   $19.90   $19.90  
  Purchase #2 $7.63   7.658%   $8.2143   $8.21   $8.20   $8.20  
  Purchase #3 $110.65   9.220%   $120.8519   $120.85   $120.85   $120.90  
  Purchase #4 $44.84   3.303%   $46.3211   $46.32   $46.30   $46.30  
  Purchase #5 $150.81   8.321%   $163.3589   $163.36   $163.35   $163.40  
  Purchase #6 $47.99   6.841%   $51.273   $51.27   $51.25   $51.30  
  Purchase #7 $209.57   8.578%   $227.5469   $227.55   $227.55   $227.50  
  7 purchase totals = $589.20       $637.4527   $637.45   $637.40   $637.50