Hi,
In case you assign tax code directly to tax G/L then you will have to enter each time same tax code for any transactions you do but sometimes it don't work as in case of TAXINN procedure as it is condition based and whenever you have to do manual adjustments suppose to these tax G/Ls SAP will not post as it will not find necessary condition.
In addition to it in case of TAXINN procedure many create only one G/L VAT account and assign tax code in condition so in this case posting will not place in case if we maintain tax code in G/L master..
Regards,
Tejas