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 $35.74   12.730%   $40.2897   $40.29   $40.30   $40.30  
  Purchase #2 $79.27   12.233%   $88.9671   $88.97   $88.95   $89.00  
  Purchase #3 $233.61   11.190%   $259.751   $259.75   $259.75   $259.80  
  Purchase #4 $9.51   8.698%   $10.3372   $10.34   $10.35   $10.30  
  Purchase #5 $18.44   4.379%   $19.2475   $19.25   $19.25   $19.20  
  Purchase #6 $224.77   12.830%   $253.608   $253.61   $253.60   $253.60  
  Purchase #7 $126.53   7.835%   $136.4436   $136.44   $136.45   $136.40  
  7 purchase totals = $727.87       $808.6441   $808.65   $808.65   $808.60