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 $36.74   8.857%   $39.9941   $39.99   $40.00   $40.00  
  Purchase #2 $42.32   10.198%   $46.6358   $46.64   $46.65   $46.60  
  Purchase #3 $155.60   12.278%   $174.7046   $174.70   $174.70   $174.70  
  Purchase #4 $27.26   13.604%   $30.9685   $30.97   $30.95   $31.00  
  Purchase #5 $178.26   12.773%   $201.0291   $201.03   $201.05   $201.00  
  Purchase #6 $20.86   13.191%   $23.6116   $23.61   $23.60   $23.60  
  Purchase #7 $150.69   5.796%   $159.424   $159.42   $159.40   $159.40  
  7 purchase totals = $611.73       $676.3677   $676.36   $676.35   $676.30