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 $26.51   13.306%   $30.0374   $30.04   $30.05   $30.00  
  Purchase #2 $87.58   9.739%   $96.1094   $96.11   $96.10   $96.10  
  Purchase #3 $219.86   6.185%   $233.4583   $233.46   $233.45   $233.50  
  Purchase #4 $132.35   4.156%   $137.8505   $137.85   $137.85   $137.90  
  Purchase #5 $86.44   4.843%   $90.6263   $90.63   $90.65   $90.60  
  Purchase #6 $243.61   4.506%   $254.5871   $254.59   $254.60   $254.60  
  Purchase #7 $116.30   7.137%   $124.6003   $124.60   $124.60   $124.60  
  7 purchase totals = $912.65       $967.2693   $967.28   $967.30   $967.30