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 $103.80   4.178%   $108.1368   $108.14   $108.15   $108.10  
  Purchase #2 $83.43   9.825%   $91.627   $91.63   $91.65   $91.60  
  Purchase #3 $227.56   13.575%   $258.4513   $258.45   $258.45   $258.50  
  Purchase #4 $76.51   13.783%   $87.0554   $87.06   $87.05   $87.10  
  Purchase #5 $7.97   13.618%   $9.0554   $9.06   $9.05   $9.10  
  Purchase #6 $22.62   5.836%   $23.9401   $23.94   $23.95   $23.90  
  Purchase #7 $154.66   6.362%   $164.4995   $164.50   $164.50   $164.50  
  7 purchase totals = $676.55       $742.7655   $742.78   $742.80   $742.80