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 $11.99   7.823%   $12.928   $12.93   $12.95   $12.90  
  Purchase #2 $224.59   6.482%   $239.1479   $239.15   $239.15   $239.10  
  Purchase #3 $89.53   3.574%   $92.7298   $92.73   $92.75   $92.70  
  Purchase #4 $38.73   6.758%   $41.3474   $41.35   $41.35   $41.30  
  Purchase #5 $129.79   6.297%   $137.9629   $137.96   $137.95   $138.00  
  Purchase #6 $50.88   11.794%   $56.8808   $56.88   $56.90   $56.90  
  Purchase #7 $170.92   12.249%   $191.856   $191.86   $191.85   $191.90  
  7 purchase totals = $716.43       $772.8528   $772.86   $772.90   $772.80