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 $34.85   12.824%   $39.3192   $39.32   $39.30   $39.30  
  Purchase #2 $238.87   8.820%   $259.9383   $259.94   $259.95   $259.90  
  Purchase #3 $48.35   13.475%   $54.8652   $54.87   $54.85   $54.90  
  Purchase #4 $119.97   4.186%   $124.9919   $124.99   $125.00   $125.00  
  Purchase #5 $61.37   10.371%   $67.7347   $67.73   $67.75   $67.70  
  Purchase #6 $218.53   4.286%   $227.8962   $227.90   $227.90   $227.90  
  Purchase #7 $46.93   9.446%   $51.363   $51.36   $51.35   $51.40  
  7 purchase totals = $768.87       $826.1085   $826.11   $826.10   $826.10