The correct opinion is that it is a Sunnah for the fasting person to use the Siwak both at the beginning of the day and at the end of it, based upon the generality of the statement of the Prophet sallAllaahu ‘alayhi wa sallam:
ÇáÓøöæÇß ãØåÑÉ ááÝã ãÑÖÇÉ ááÑÈøö
”The Siwak purifies the mouth and it is pleasing to the Lord.” 
He sallAllaahu ‘alayhi wa sallam also said:
áæ áÇ Ãä ÃóÔÞ Úáì ÃñãÊí áÃóãÑÊåã ÈÇáÓøöæÇß ãÚ ßá ÕáÇÉò
”If it were not that it would burden be a burden upon my people, I would have ordered them to use the Siwak before every prayer.” 
ööAs for perfume, likewise, it is permissible for the fasting person at the start of the day and at the end of it, whether the perfume is incense, oil or anything else. But it is not permissible for him to sniff the incense, because incense has particles which can be felt and seen, and if he sniffs it, it will rise up into his nose and then to his stomach, which is why the Prophet sallAllaahu ‘alayhi wa sallam said to Laqit bin Sabirah, may Allaah be pleased with him:
ÈÇáÛ Ýí ÇáÇÓÊäÔÇÞ ÅöáÇ Ãä Êßæä ÕÇÆãÇ
”Exaggerate in sniffing water into the nose (i.e. during ablution) except when you are fasting.” 
 Siwak: A twig taken from the roots of the arak tree and used as a toothbrush.
 Reported by al-Bukhari in the Book of Fasting, in the Chapter: The Moist and Dry Siwak for the Fasting Person.
 Reported by al-Bukhari in the Book of Fasting, in the Chapter: The Moist and Dry Siwak for the Fasting Person and by Muslim in the Book of Purification, in the Chapter: The Siwak (252).
 Reported by Abu Dawud in the Book of Istinthar (Blowing Water Out of the Nose) (142); by At-Tirmithi in the Book of Purification, in the Chapter: What Has Been Said Regarding Washing Between the Toes (38); by An-Nasa’I in the Book of Purification, in the Chapter: Istinshaq (Sniffing Water Through the Nose) to the highest Degree (87); and by Ibn Majah in the Book of Purification and its Sunan, in the Chapter: Washing Between the Fingers and Toes (448).