You can define Default sale tax as well as Purchase tax in openerp 7.
That will be set at the product when you create new product than it will automatically set default sale tax as well as purchase tax.
...Enjoy...
import xmlrpclib
username = 'admin'
password = 'a'
database = 'test_60A'
sock_comm = xmlrpclib.ServerProxy('http://localhost:8069/xmlrpc/common')
uid = sock_comm.login(database,username,password)
sock = xmlrpclib.ServerProxy('http://localhost:8069/xmlrpc/object')
print "connection success full"
ids = sock.execute(database,uid, password,'res.company', 'search', [])
print "list of ids",ids
#ids will contain ids of company A and B
sock.execute(database,uid, password, 'res.company', 'write', ids, {'rml_header1': 'hop'})