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 $227.91   13.323%   $258.2744   $258.27   $258.25   $258.30  
  Purchase #2 $154.74   6.378%   $164.6093   $164.61   $164.60   $164.60  
  Purchase #3 $78.67   9.852%   $86.4206   $86.42   $86.40   $86.40  
  Purchase #4 $25.86   5.338%   $27.2404   $27.24   $27.25   $27.20  
  Purchase #5 $189.94   6.285%   $201.8777   $201.88   $201.90   $201.90  
  Purchase #6 $183.55   3.679%   $190.3028   $190.30   $190.30   $190.30  
  Purchase #7 $20.36   12.492%   $22.9034   $22.90   $22.90   $22.90  
  7 purchase totals = $881.03       $951.6286   $951.62   $951.60   $951.60