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 $18.58   7.778%   $20.0252   $20.03   $20.05   $20.00  
  Purchase #2 $10.87   9.787%   $11.9338   $11.93   $11.95   $11.90  
  Purchase #3 $235.77   4.128%   $245.5026   $245.50   $245.50   $245.50  
  Purchase #4 $41.35   12.236%   $46.4096   $46.41   $46.40   $46.40  
  Purchase #5 $106.77   7.564%   $114.8461   $114.85   $114.85   $114.80  
  Purchase #6 $19.41   11.842%   $21.7085   $21.71   $21.70   $21.70  
  Purchase #7 $176.41   6.138%   $187.238   $187.24   $187.25   $187.20  
  7 purchase totals = $609.16       $647.6638   $647.67   $647.70   $647.50