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 $23.78   12.441%   $26.7385   $26.74   $26.75   $26.70  
  Purchase #2 $124.97   6.857%   $133.5392   $133.54   $133.55   $133.50  
  Purchase #3 $225.54   7.621%   $242.7284   $242.73   $242.75   $242.70  
  Purchase #4 $177.92   11.439%   $198.2723   $198.27   $198.25   $198.30  
  Purchase #5 $23.56   13.393%   $26.7154   $26.72   $26.70   $26.70  
  Purchase #6 $212.29   6.599%   $226.299   $226.30   $226.30   $226.30  
  Purchase #7 $116.30   7.766%   $125.3319   $125.33   $125.35   $125.30  
  7 purchase totals = $904.36       $979.6247   $979.63   $979.65   $979.50