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 $28.70   13.272%   $32.5091   $32.51   $32.50   $32.50  
  Purchase #2 $7.88   10.102%   $8.676   $8.68   $8.70   $8.70  
  Purchase #3 $224.79   11.553%   $250.76   $250.76   $250.75   $250.80  
  Purchase #4 $62.91   8.544%   $68.285   $68.29   $68.30   $68.30  
  Purchase #5 $30.41   9.606%   $33.3312   $33.33   $33.35   $33.30  
  Purchase #6 $205.88   5.400%   $216.9975   $217.00   $217.00   $217.00  
  Purchase #7 $79.44   13.759%   $90.3701   $90.37   $90.35   $90.40  
  7 purchase totals = $640.01       $700.9289   $700.94   $700.95   $701.00