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 $94.85   3.529%   $98.1973   $98.20   $98.20   $98.20  
  Purchase #2 $187.46   3.440%   $193.9086   $193.91   $193.90   $193.90  
  Purchase #3 $137.72   12.211%   $154.537   $154.54   $154.55   $154.50  
  Purchase #4 $209.27   12.193%   $234.7863   $234.79   $234.80   $234.80  
  Purchase #5 $35.54   11.442%   $39.6065   $39.61   $39.60   $39.60  
  Purchase #6 $183.50   3.388%   $189.717   $189.72   $189.70   $189.70  
  Purchase #7 $78.58   4.716%   $82.2858   $82.29   $82.30   $82.30  
  7 purchase totals = $926.92       $993.0385   $993.06   $993.05   $993.00