I used to underlet all the time, but it is slower than just Dumping the grain straight into the mash tun with water already in.
The underletting used to take about 10minutes for me and because my mash water was usually around 74-75c; in theory the grain that first came into contact with the mash water ran the risk of denaturing the enzymes (at the bottom of the mash tun) due to thermal mass temp.
I simply now heat my mash water to 74-75c, transfer to the mash tun then feed the grain into the mash tun usually around the edge first and slowly mix. The whole process takes about 30secs; I do stir until all the grain is soaked and mixed up and haven't had any dough balling. A couple of minutes and all done. I then stir every 20minutes or so to aid conversion.
Simple!
AC