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 $234.76   13.371%   $266.1498   $266.15   $266.15   $266.10  
  Purchase #2 $173.64   12.108%   $194.6643   $194.66   $194.65   $194.70  
  Purchase #3 $153.39   5.829%   $162.3311   $162.33   $162.35   $162.30  
  Purchase #4 $34.94   8.172%   $37.7953   $37.80   $37.80   $37.80  
  Purchase #5 $35.68   5.725%   $37.7227   $37.72   $37.70   $37.70  
  Purchase #6 $129.90   5.612%   $137.19   $137.19   $137.20   $137.20  
  Purchase #7 $13.29   8.351%   $14.3998   $14.40   $14.40   $14.40  
  7 purchase totals = $775.60       $850.2530   $850.25   $850.25   $850.20