"Amen" (or its linguistic equivalent) is used in many different ways and on a lots of occasions in the Scriptures. You can get a short overview at
What Does Amen Mean? Bible Definition .
In I think the majority of cases, there are quite a few people around, being addressed, or involved in the conversation. The word is often meant to convey the idea that everyone is on board, everyone agrees, that they all think "it should be done" - "so be it." When seen as a response of listeners to someone's prayer, or statement, and showing agreement with the prayer/statement, the idea that I "need" to end my personal prayer with "Amen" or the longer "in Jesus name, Amen" is usually excessive. It would suggest that you are agreeing with yourself, that you really mean what you say (why would you say things to the Almighty and Omniscient if you didn't mean it? You cannot deceive God).
IMHO, if you're praying to Jesus, your Lord, it is odd to say "in Jesus name" - after all you're talk to/with Him. But if your prayer is addressed to your "Heavenly Father," praying "in Jesus name" is certainly appropriate (assuming you have a fair idea of the implications) and valuable - Jesus is representing you in heaven, and the Holy Spirit is conveying the prayer of your heart, to the Father, and He always seeks to glorify the Son - Jesus.
If you are praying out loud in a group, it is highly appropriate for everyone who agrees with your prayer to say "amen" - not only at the end, but at any time you say something that they feel strongly in favour of. ("if two of you on earth agree about anything they ask for, it will be done for them by my Father in heaven." Matthew 18:19) Amen in such a situation is equivalent to letting the Lord/God know explicitly that you agree with the one actually voicing the prayer.