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 $14.55   9.248%   $15.8956   $15.90   $15.90   $15.90  
  Purchase #2 $182.25   9.449%   $199.4708   $199.47   $199.45   $199.50  
  Purchase #3 $26.69   8.560%   $28.9747   $28.97   $28.95   $29.00  
  Purchase #4 $122.33   8.465%   $132.6852   $132.69   $132.70   $132.70  
  Purchase #5 $75.41   5.596%   $79.6299   $79.63   $79.65   $79.60  
  Purchase #6 $122.90   12.445%   $138.1949   $138.19   $138.20   $138.20  
  Purchase #7 $44.89   12.366%   $50.4411   $50.44   $50.45   $50.40  
  7 purchase totals = $589.02       $645.2922   $645.29   $645.30   $645.30