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 $8.79   12.457%   $9.885   $9.89   $9.90   $9.90  
  Purchase #2 $18.54   11.643%   $20.6986   $20.70   $20.70   $20.70  
  Purchase #3 $199.82   11.830%   $223.4587   $223.46   $223.45   $223.50  
  Purchase #4 $241.49   10.860%   $267.7158   $267.72   $267.70   $267.70  
  Purchase #5 $193.54   7.608%   $208.2645   $208.26   $208.25   $208.30  
  Purchase #6 $248.73   13.553%   $282.4404   $282.44   $282.45   $282.40  
  Purchase #7 $180.65   3.452%   $186.886   $186.89   $186.90   $186.90  
  7 purchase totals = $1,091.56       $1,199.3490   $1,199.36   $1,199.35   $1,199.40