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 $128.31   6.462%   $136.6014   $136.60   $136.60   $136.60  
  Purchase #2 $194.80   4.842%   $204.2322   $204.23   $204.25   $204.20  
  Purchase #3 $56.25   13.613%   $63.9073   $63.91   $63.90   $63.90  
  Purchase #4 $94.70   3.245%   $97.773   $97.77   $97.75   $97.80  
  Purchase #5 $167.37   7.213%   $179.4424   $179.44   $179.45   $179.40  
  Purchase #6 $142.80   5.707%   $150.9496   $150.95   $150.95   $150.90  
  Purchase #7 $186.54   5.327%   $196.477   $196.48   $196.50   $196.50  
  7 purchase totals = $970.77       $1,029.3829   $1,029.38   $1,029.40   $1,029.30