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 $206.69   13.338%   $234.2583   $234.26   $234.25   $234.30  
  Purchase #2 $158.91   8.206%   $171.9502   $171.95   $171.95   $172.00  
  Purchase #3 $79.63   9.164%   $86.9273   $86.93   $86.95   $86.90  
  Purchase #4 $141.83   8.693%   $154.1593   $154.16   $154.15   $154.20  
  Purchase #5 $188.91   7.167%   $202.4492   $202.45   $202.45   $202.40  
  Purchase #6 $157.26   11.492%   $175.3323   $175.33   $175.35   $175.30  
  Purchase #7 $57.66   9.649%   $63.2236   $63.22   $63.20   $63.20  
  7 purchase totals = $990.89       $1,088.3002   $1,088.30   $1,088.30   $1,088.30