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 $245.83   13.596%   $279.253   $279.25   $279.25   $279.30  
  Purchase #2 $55.55   5.493%   $58.6014   $58.60   $58.60   $58.60  
  Purchase #3 $78.28   4.491%   $81.7956   $81.80   $81.80   $81.80  
  Purchase #4 $212.45   6.709%   $226.7033   $226.70   $226.70   $226.70  
  Purchase #5 $201.64   6.225%   $214.1921   $214.19   $214.20   $214.20  
  Purchase #6 $128.25   9.392%   $140.2952   $140.30   $140.30   $140.30  
  Purchase #7 $17.46   4.252%   $18.2024   $18.20   $18.20   $18.20  
  7 purchase totals = $939.46       $1,019.0430   $1,019.04   $1,019.05   $1,019.10