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 $5.72   13.414%   $6.4873   $6.49   $6.50   $6.50  
  Purchase #2 $103.81   5.102%   $109.1064   $109.11   $109.10   $109.10  
  Purchase #3 $74.99   12.302%   $84.2153   $84.22   $84.20   $84.20  
  Purchase #4 $50.45   7.435%   $54.201   $54.20   $54.20   $54.20  
  Purchase #5 $179.90   3.120%   $185.5129   $185.51   $185.50   $185.50  
  Purchase #6 $91.82   4.175%   $95.6535   $95.65   $95.65   $95.70  
  Purchase #7 $97.78   13.602%   $111.08   $111.08   $111.10   $111.10  
  7 purchase totals = $604.47       $646.2564   $646.26   $646.25   $646.30