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 $54.99   9.842%   $60.4021   $60.40   $60.40   $60.40  
  Purchase #2 $244.49   5.634%   $258.2646   $258.26   $258.25   $258.30  
  Purchase #3 $19.72   4.377%   $20.5831   $20.58   $20.60   $20.60  
  Purchase #4 $47.87   13.713%   $54.4344   $54.43   $54.45   $54.40  
  Purchase #5 $186.60   8.663%   $202.7652   $202.77   $202.75   $202.80  
  Purchase #6 $250.25   3.818%   $259.8045   $259.80   $259.80   $259.80  
  Purchase #7 $21.45   5.709%   $22.6746   $22.67   $22.65   $22.70  
  7 purchase totals = $825.37       $878.9285   $878.91   $878.90   $879.00