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 $5.94   7.124%   $6.3632   $6.36   $6.35   $6.40  
  Purchase #2 $168.39   6.463%   $179.273   $179.27   $179.25   $179.30  
  Purchase #3 $15.49   8.173%   $16.756   $16.76   $16.75   $16.80  
  Purchase #4 $128.54   6.675%   $137.12   $137.12   $137.10   $137.10  
  Purchase #5 $22.81   13.149%   $25.8093   $25.81   $25.80   $25.80  
  Purchase #6 $221.39   3.123%   $228.304   $228.30   $228.30   $228.30  
  Purchase #7 $43.50   7.773%   $46.8813   $46.88   $46.90   $46.90  
  7 purchase totals = $606.06       $640.5068   $640.50   $640.45   $640.60