The gift of tongues is some what multifaceted in that it has many uses and can be used in a number of ways. It can be used to minister to others and to self. It can edify yourself, another person or a whole church. Depending upon the way it is directed, and the will of the Holy Spirit it can be just as edifying as prophecy, so long as there is an interpreter.
Some of the things that can be done with the gift of tongues: Intercession, singing in the Spirit, praying in the Spirit, giving a word from the Lord to minister to another, and even speaking in a foreign tongue in someone else's own language that you have no knowledge of. It also can be used to fellowship with the Lord, both privately between oneself and the Lord or with an interpreter. Along these same lines, the Lord Himself can speak through the gift and it not just be your spirit speaking to the Father (in prayer). He, Himself, can speak through the gift both privately and corporately.
In a public setting one ought to have the decency to only speak when an interpreter is present, or as the Holy Spirit leads. You may not be aware that an interpreter is present, but the Lord knows. Also, as far as decency goes its a matter of not causing fellow brothers and sisters to stumble and to respect that you may be distracting them from their own worship. Keep in mind an interpreter doesn't have to be present because you, yourself, can interpret your own tongues as the word of God says to the interpreter to pray that they may interpret.
It can be done in order, and ought to. As far as mandatory is concerned one must keep in mind that not all tongues are interpreted or revealed. It is up to the discretion of the Holy Spirit. Sometimes it may be a shielded prayer or intercession for something you are not to yet know of.
I suppose in this respect you must build a reputation as a tongue speaker so that others will trust that what you have spoken is from the Lord, regardless of interpretation. Any time a genuine tongue is spoken its from the Lord, just because it isn't interpreted doesn't mean its not from God. The gift is volitional in that the speaker can do it at will, and it is the Holy Spirit giving utterance to your spirit. To discredit the tongue because its not interpreted is incorrect, but in so far as your mind being fruitful of it you can just be reassured that the Holy Spirit has guided the intercession.
If you are addressing the church, giving a corporate message, then yes it is mandatory. VVhat edification would happen otherwise? If you wish to intercede for the church in tongues, by all means do it. Even so, under your breath if in the midst of people. You don't need to make a show of it, but can do it gracefully and respectfully.