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 $7.77   4.647%   $8.1311   $8.13   $8.15   $8.10  
  Purchase #2 $129.45   6.189%   $137.4617   $137.46   $137.45   $137.50  
  Purchase #3 $198.82   11.548%   $221.7797   $221.78   $221.80   $221.80  
  Purchase #4 $27.46   12.784%   $30.9705   $30.97   $30.95   $31.00  
  Purchase #5 $12.48   6.356%   $13.2732   $13.27   $13.25   $13.30  
  Purchase #6 $181.73   3.841%   $188.7102   $188.71   $188.70   $188.70  
  Purchase #7 $36.88   12.696%   $41.5623   $41.56   $41.55   $41.60  
  7 purchase totals = $594.59       $641.8887   $641.88   $641.85   $642.00