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 $225.41   9.371%   $246.5332   $246.53   $246.55   $246.50  
  Purchase #2 $58.91   4.619%   $61.6311   $61.63   $61.65   $61.60  
  Purchase #3 $24.43   10.154%   $26.9106   $26.91   $26.90   $26.90  
  Purchase #4 $134.40   9.515%   $147.1882   $147.19   $147.20   $147.20  
  Purchase #5 $56.60   8.215%   $61.2497   $61.25   $61.25   $61.20  
  Purchase #6 $48.68   5.299%   $51.2596   $51.26   $51.25   $51.30  
  Purchase #7 $48.34   13.598%   $54.9133   $54.91   $54.90   $54.90  
  7 purchase totals = $596.77       $649.6857   $649.68   $649.70   $649.60