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 $69.78   13.584%   $79.2589   $79.26   $79.25   $79.30  
  Purchase #2 $68.34   9.277%   $74.6799   $74.68   $74.70   $74.70  
  Purchase #3 $95.99   4.365%   $100.18   $100.18   $100.20   $100.20  
  Purchase #4 $89.47   9.140%   $97.6476   $97.65   $97.65   $97.60  
  Purchase #5 $174.53   12.675%   $196.6517   $196.65   $196.65   $196.70  
  Purchase #6 $85.26   7.834%   $91.9393   $91.94   $91.95   $91.90  
  Purchase #7 $51.78   8.824%   $56.3491   $56.35   $56.35   $56.30  
  7 purchase totals = $635.15       $696.7065   $696.71   $696.75   $696.70