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 $108.63   13.144%   $122.9083   $122.91   $122.90   $122.90  
  Purchase #2 $126.96   7.811%   $136.8768   $136.88   $136.90   $136.90  
  Purchase #3 $114.45   8.311%   $123.9619   $123.96   $123.95   $124.00  
  Purchase #4 $220.37   7.770%   $237.4927   $237.49   $237.50   $237.50  
  Purchase #5 $119.95   10.810%   $132.9166   $132.92   $132.90   $132.90  
  Purchase #6 $240.68   9.396%   $263.2943   $263.29   $263.30   $263.30  
  Purchase #7 $11.44   12.291%   $12.8461   $12.85   $12.85   $12.80  
  7 purchase totals = $942.48       $1,030.2967   $1,030.30   $1,030.30   $1,030.30