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 $15.73   13.690%   $17.8834   $17.88   $17.90   $17.90  
  Purchase #2 $104.54   7.618%   $112.5039   $112.50   $112.50   $112.50  
  Purchase #3 $138.92   13.716%   $157.9743   $157.97   $157.95   $158.00  
  Purchase #4 $130.89   7.573%   $140.8023   $140.80   $140.80   $140.80  
  Purchase #5 $47.38   5.369%   $49.9238   $49.92   $49.90   $49.90  
  Purchase #6 $142.75   8.685%   $155.1478   $155.15   $155.15   $155.10  
  Purchase #7 $52.68   5.261%   $55.4515   $55.45   $55.45   $55.50  
  7 purchase totals = $632.89       $689.6870   $689.67   $689.65   $689.70