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 $230.94   12.841%   $260.595   $260.60   $260.60   $260.60  
  Purchase #2 $60.87   6.147%   $64.6117   $64.61   $64.60   $64.60  
  Purchase #3 $248.39   10.448%   $274.3418   $274.34   $274.35   $274.30  
  Purchase #4 $250.32   11.272%   $278.5361   $278.54   $278.55   $278.50  
  Purchase #5 $15.68   10.847%   $17.3808   $17.38   $17.40   $17.40  
  Purchase #6 $209.78   12.591%   $236.1934   $236.19   $236.20   $236.20  
  Purchase #7 $21.48   11.629%   $23.9779   $23.98   $24.00   $24.00  
  7 purchase totals = $1,037.46       $1,155.6367   $1,155.64   $1,155.70   $1,155.60