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 $249.53   7.783%   $268.9509   $268.95   $268.95   $269.00  
  Purchase #2 $5.89   4.686%   $6.166   $6.17   $6.15   $6.20  
  Purchase #3 $110.89   5.828%   $117.3527   $117.35   $117.35   $117.40  
  Purchase #4 $83.89   9.823%   $92.1305   $92.13   $92.15   $92.10  
  Purchase #5 $127.27   6.653%   $135.7373   $135.74   $135.75   $135.70  
  Purchase #6 $88.47   6.403%   $94.1347   $94.13   $94.15   $94.10  
  Purchase #7 $117.26   8.538%   $127.2717   $127.27   $127.25   $127.30  
  7 purchase totals = $783.20       $841.7438   $841.74   $841.75   $841.80