The tag you added is for Yoga-Asana. But your question seems to be about seats.
I am answering for both.
Manufactured seats, like those made of wool, Kusha etc, are needed.
Seats made of wool, blanket etc are good to use. On the other hand, seats made of wood etc should be abandoned.
The wise must reject seats (for Japa Dhyana purpose) made of bamboo,
stone, earth, wood, grass and leaves. The reason is, these seats are
giver of poverty, misery and diseases.
Tulakambala vastrAnAm simha vyAghra mrigAjinam |
dhimAn soubhAgya jyAna vriddhidam ||
The wise person must use seats which are made of wool, blanket, cloth,
tiger-skin and deer-skin. These types of seat give good fortune and
KulArnava Tantram 15.33,34
Most people in today's time use the seats made of Kusha or wool. Seats made of deer-skin etc are of course illegal to be used now.
Yogic posture (Asana):
This is also required to attain success in mantra japa and allied activities like PUjA, DhyAna etc.
Padmaswastika virAdisvAsaneshupavishya cha |
kuryAd anyathA nishphalam bhavet ||
One must perform Japa-PujA and other such spiritual activities only while
being seated in one of the Yogic postures like Padma Asana, Swastika
Asana, Vira Asana etc. Otherwise the acts do not bear any fruits
The book Nityotsava states that before starting the worship the devotee must secure his position in Padma Asana.
Swami Sivananda also advises his disciples in the same way:
Select any Mantra or Name of God, preferably that given to you by your
Guru, and repeat it from 108 to 1,080 times daily (one to ten malas).
Always keep your Guru-Mantra a secret. Never disclose it to anyone.
It is better to stick to one Mantra only. See Lord Krishna in Rama,
Shiva, Durga, Gayatri and in everyone.
Get up at 4a.m. and do Japa for two hours. The early morning period
(Brahmamuhurta) and dusk is the most favourable time for Japa and
meditation. This is when Sattva (purity or steadiness) is predominant.
Take a bath or wash your hands, feet, face and mouth before sitting
for Japa in the morning. At other times this is not absolutely
necessary. Do Japa whenever you have leisure, at the three junctions
of the day - morning, noon and evening - and before going to bed.
Face east or north during the practice. This enhances the efficacy of
the Japa. Sit on a deer skin or rug. Spread a piece of cloth over it.
This conserves body-electricity.
Sit in a separate meditation room or in any suitable place, such as a
temple, on a river bank or under a banyan or peepul tree. Maintain a
steady pose. Attain mastery of the posture. You must be able to sit
in Padmasana, Siddhasana or Sukhasana for three hours at a strech.
Should we always use an Asana in Puja-Japa etc?
Yes, we must. To stress on the fact, Lord Shiva in Gauri YAmala Tantram says:
Salile yadi krvita devatAnAm prapujanam |
notthitantu tathAchAret ||
Even if worship is done in water, one should do it while being seated
upon on an Asana; never worship by getting up from the Asana.
Needless to say, in water, we can not use a seat. It has to be mentally imagined. But Asana is a must in all circumstances. And, to emphasize on that the above verse is being said.
Asanas that are good to be used and the benefits in using them:
Already mentioned previously. Here are a few more verses from Sammohana Tantram:
VastrAsane vyAdhinAshah kambale dukkha-nAshanam ||
Use of Vastra-asana destroys diseases and use of blanket made Asana
leads to destruction of misery.
KushAsane bhaved Ayur mokshah syAd vyAghracharmAni |
bhavet putri kamabale siddhiruttama ||
ShAntike dhavalah proktah
SyAt poushtike tu kausheyam kambale
Using Kusha Asana increases lifespan; using tiger-skin made Asana
gives liberation; Deer-skin made Asana is a giver of progeny and
blanket-made Asana gives excellent Siddhis.
Different Asanas for different deities?
Yes, see the following Gandharva Tantram verses:
Kambalam charmaja chailam mahAmAyA-prapujane ||
proktam kAmAkhyAyAs tathaiva cha |
vishnoshchApi kushAsanam ||
TrinAsane yasohAnih pallave
YathoktamasAnam kuryAt sarva-siddhi-pradAyakam
Na yatheshtosano bhuyAt pujAkarmani sAdhakam ||
In worship of MahAmAyA and KAmAkhyA, seats made of blanket, leather
and Chaila (clothes made) are prescribed. And, in worship of TripurA, Rudra and
Vishnu, Kusha Asana is auspicious. Grass-made seats bestow destruction
of fame and those made of leaves give rise to delusion in mind. The
devotee must thus select the most appropriate seats. And, the choice
must not be based on one's whims.
The Gauri YAmala Tantram, Sammohana Tantram and Gandharva Tantram's verses are all quoted in Shakta Ananda Tarangini's 5th Chapter, under the section "Asana Nirnaya".