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 $246.49   13.412%   $279.5492   $279.55   $279.55   $279.50  
  Purchase #2 $166.41   12.147%   $186.6238   $186.62   $186.60   $186.60  
  Purchase #3 $11.53   11.307%   $12.8337   $12.83   $12.85   $12.80  
  Purchase #4 $61.58   3.208%   $63.5555   $63.56   $63.55   $63.60  
  Purchase #5 $162.26   11.514%   $180.9426   $180.94   $180.95   $180.90  
  Purchase #6 $202.99   10.335%   $223.969   $223.97   $223.95   $224.00  
  Purchase #7 $116.37   3.305%   $120.216   $120.22   $120.20   $120.20  
  7 purchase totals = $967.63       $1,067.6898   $1,067.69   $1,067.65   $1,067.60