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 $121.98   11.486%   $135.9906   $135.99   $136.00   $136.00  
  Purchase #2 $188.50   10.807%   $208.8712   $208.87   $208.85   $208.90  
  Purchase #3 $134.42   13.244%   $152.2226   $152.22   $152.20   $152.20  
  Purchase #4 $119.84   11.551%   $133.6827   $133.68   $133.70   $133.70  
  Purchase #5 $128.83   5.692%   $136.163   $136.16   $136.15   $136.20  
  Purchase #6 $231.47   9.801%   $254.1564   $254.16   $254.15   $254.20  
  Purchase #7 $214.48   11.536%   $239.2224   $239.22   $239.20   $239.20  
  7 purchase totals = $1,139.52       $1,260.3089   $1,260.30   $1,260.25   $1,260.40