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 $116.98   12.680%   $131.8131   $131.81   $131.80   $131.80  
  Purchase #2 $176.45   13.691%   $200.6078   $200.61   $200.60   $200.60  
  Purchase #3 $161.36   8.210%   $174.6077   $174.61   $174.60   $174.60  
  Purchase #4 $160.56   7.474%   $172.5603   $172.56   $172.55   $172.60  
  Purchase #5 $117.37   12.629%   $132.1927   $132.19   $132.20   $132.20  
  Purchase #6 $190.81   3.692%   $197.8547   $197.85   $197.85   $197.90  
  Purchase #7 $226.71   9.391%   $248.0003   $248.00   $248.00   $248.00  
  7 purchase totals = $1,150.24       $1,257.6366   $1,257.63   $1,257.60   $1,257.70