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 $229.92   10.707%   $254.5375   $254.54   $254.55   $254.50  
  Purchase #2 $25.48   5.188%   $26.8019   $26.80   $26.80   $26.80  
  Purchase #3 $68.84   7.654%   $74.109   $74.11   $74.10   $74.10  
  Purchase #4 $60.73   3.809%   $63.0432   $63.04   $63.05   $63.00  
  Purchase #5 $60.25   4.841%   $63.1667   $63.17   $63.15   $63.20  
  Purchase #6 $215.35   8.401%   $233.4416   $233.44   $233.45   $233.40  
  Purchase #7 $114.83   8.689%   $124.8076   $124.81   $124.80   $124.80  
  7 purchase totals = $775.40       $839.9075   $839.91   $839.90   $839.80