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 $10.94   12.761%   $12.3361   $12.34   $12.35   $12.30  
  Purchase #2 $118.97   12.856%   $134.2648   $134.26   $134.25   $134.30  
  Purchase #3 $239.56   12.537%   $269.5936   $269.59   $269.60   $269.60  
  Purchase #4 $23.87   3.428%   $24.6883   $24.69   $24.70   $24.70  
  Purchase #5 $76.61   10.294%   $84.4962   $84.50   $84.50   $84.50  
  Purchase #6 $163.72   13.682%   $186.1202   $186.12   $186.10   $186.10  
  Purchase #7 $62.60   9.518%   $68.5583   $68.56   $68.55   $68.60  
  7 purchase totals = $696.27       $780.0575   $780.06   $780.05   $780.10