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 $206.49   6.816%   $220.5644   $220.56   $220.55   $220.60  
  Purchase #2 $126.76   11.155%   $140.9001   $140.90   $140.90   $140.90  
  Purchase #3 $16.62   11.487%   $18.5291   $18.53   $18.55   $18.50  
  Purchase #4 $250.99   13.510%   $284.8987   $284.90   $284.90   $284.90  
  Purchase #5 $189.27   10.295%   $208.7553   $208.76   $208.75   $208.80  
  Purchase #6 $23.47   13.244%   $26.5784   $26.58   $26.60   $26.60  
  Purchase #7 $129.83   3.208%   $133.9949   $133.99   $134.00   $134.00  
  7 purchase totals = $943.43       $1,034.2209   $1,034.22   $1,034.25   $1,034.30