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 $18.99   5.350%   $20.006   $20.01   $20.00   $20.00  
  Purchase #2 $124.41   6.825%   $132.901   $132.90   $132.90   $132.90  
  Purchase #3 $72.40   13.358%   $82.0712   $82.07   $82.05   $82.10  
  Purchase #4 $21.39   11.631%   $23.8779   $23.88   $23.90   $23.90  
  Purchase #5 $210.90   10.654%   $233.3693   $233.37   $233.35   $233.40  
  Purchase #6 $193.61   9.798%   $212.5799   $212.58   $212.60   $212.60  
  Purchase #7 $17.79   12.383%   $19.9929   $19.99   $20.00   $20.00  
  7 purchase totals = $659.49       $724.7982   $724.80   $724.80   $724.90