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 $245.44   12.702%   $276.6158   $276.62   $276.60   $276.60  
  Purchase #2 $110.92   3.636%   $114.9531   $114.95   $114.95   $115.00  
  Purchase #3 $132.30   11.795%   $147.9048   $147.90   $147.90   $147.90  
  Purchase #4 $41.63   11.395%   $46.3737   $46.37   $46.35   $46.40  
  Purchase #5 $32.58   13.479%   $36.9715   $36.97   $36.95   $37.00  
  Purchase #6 $6.59   12.242%   $7.3967   $7.40   $7.40   $7.40  
  Purchase #7 $25.46   3.242%   $26.2854   $26.29   $26.30   $26.30  
  7 purchase totals = $594.92       $656.5010   $656.50   $656.45   $656.60