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 $36.86   5.773%   $38.9879   $38.99   $39.00   $39.00  
  Purchase #2 $42.61   3.427%   $44.0702   $44.07   $44.05   $44.10  
  Purchase #3 $92.72   8.256%   $100.375   $100.38   $100.40   $100.40  
  Purchase #4 $140.46   11.513%   $156.6312   $156.63   $156.65   $156.60  
  Purchase #5 $238.54   8.492%   $258.7968   $258.80   $258.80   $258.80  
  Purchase #6 $5.80   4.351%   $6.0524   $6.05   $6.05   $6.10  
  Purchase #7 $176.29   3.743%   $182.8885   $182.89   $182.90   $182.90  
  7 purchase totals = $733.28       $787.8020   $787.81   $787.85   $787.90