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 $37.31   8.255%   $40.3899   $40.39   $40.40   $40.40  
  Purchase #2 $109.32   3.484%   $113.1287   $113.13   $113.15   $113.10  
  Purchase #3 $157.83   3.374%   $163.1552   $163.16   $163.15   $163.20  
  Purchase #4 $130.75   3.376%   $135.1641   $135.16   $135.15   $135.20  
  Purchase #5 $233.60   10.334%   $257.7402   $257.74   $257.75   $257.70  
  Purchase #6 $78.35   9.139%   $85.5104   $85.51   $85.50   $85.50  
  Purchase #7 $152.60   12.726%   $172.0199   $172.02   $172.00   $172.00  
  7 purchase totals = $899.76       $967.1084   $967.11   $967.10   $967.10