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 $212.41   12.797%   $239.5921   $239.59   $239.60   $239.60  
  Purchase #2 $174.51   7.308%   $187.2632   $187.26   $187.25   $187.30  
  Purchase #3 $77.28   7.373%   $82.9779   $82.98   $83.00   $83.00  
  Purchase #4 $123.33   7.518%   $132.6019   $132.60   $132.60   $132.60  
  Purchase #5 $73.49   12.431%   $82.6255   $82.63   $82.65   $82.60  
  Purchase #6 $38.68   7.476%   $41.5717   $41.57   $41.55   $41.60  
  Purchase #7 $229.38   8.325%   $248.4759   $248.48   $248.50   $248.50  
  7 purchase totals = $929.08       $1,015.1082   $1,015.11   $1,015.15   $1,015.20