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 $109.63   9.586%   $120.1391   $120.14   $120.15   $120.10  
  Purchase #2 $18.69   3.866%   $19.4126   $19.41   $19.40   $19.40  
  Purchase #3 $20.61   11.817%   $23.0455   $23.05   $23.05   $23.00  
  Purchase #4 $135.32   6.818%   $144.5461   $144.55   $144.55   $144.50  
  Purchase #5 $84.85   6.192%   $90.1039   $90.10   $90.10   $90.10  
  Purchase #6 $152.86   3.765%   $158.6152   $158.62   $158.60   $158.60  
  Purchase #7 $115.46   12.157%   $129.4965   $129.50   $129.50   $129.50  
  7 purchase totals = $637.42       $685.3589   $685.37   $685.35   $685.20