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 $23.73   12.307%   $26.6505   $26.65   $26.65   $26.70  
  Purchase #2 $112.70   5.624%   $119.0382   $119.04   $119.05   $119.00  
  Purchase #3 $177.28   6.437%   $188.6915   $188.69   $188.70   $188.70  
  Purchase #4 $63.56   10.140%   $70.005   $70.01   $70.00   $70.00  
  Purchase #5 $21.36   9.217%   $23.3288   $23.33   $23.35   $23.30  
  Purchase #6 $79.46   3.822%   $82.497   $82.50   $82.50   $82.50  
  Purchase #7 $82.79   8.828%   $90.0987   $90.10   $90.10   $90.10  
  7 purchase totals = $560.88       $600.3097   $600.32   $600.35   $600.30