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 $9.57   6.405%   $10.183   $10.18   $10.20   $10.20  
  Purchase #2 $29.29   3.505%   $30.3166   $30.32   $30.30   $30.30  
  Purchase #3 $23.29   12.246%   $26.1421   $26.14   $26.15   $26.10  
  Purchase #4 $186.80   9.528%   $204.5983   $204.60   $204.60   $204.60  
  Purchase #5 $20.31   10.665%   $22.4761   $22.48   $22.50   $22.50  
  Purchase #6 $220.56   10.830%   $244.4466   $244.45   $244.45   $244.40  
  Purchase #7 $5.25   11.721%   $5.8654   $5.87   $5.85   $5.90  
  7 purchase totals = $495.07       $544.0281   $544.04   $544.05   $544.00