Having said all that rules are made to be broken. Above all we should be sensitive to the leading of the Holy Spirit and sensitive to the congregation. It is the work of the Holy Spirit to energise the songs that we are singing and the congregation are gathered because they love Him and want to meet Him, to catch a glimpse of Him.
If a song is simple and has only 4 lines then I would use it wherever it felt appropriate. I would sing it once and then invite the congregation to join me.
Another great place to introduce new songs is in a ministry time although you still need to be sensitive to what ‘s going on around you. Not everyone will be having ministry many will still be worshipping. Just sing the song straight through, if there are lots of people still worshipping sing the first verse and chorus twice, just to give the congregation the opportunity to familiarise themselves with the song and join in.
In a conference setting I would be more inclined to introduce new songs from the first set and then use them throughout the conference. I wouldn’t spend time teaching them to the delegates. If you use the new songs consistently during the conference people will pick them up and make them their own.
How many times do we come away from a conference with new songs in our hearts.