Water Heaters - Boot tee or not boot tee

I am getting ready to install a tankless water heater in my laundry room. It is a single story house and closest wall goes to my patio which would be to low to vent and I want to enclose some day. The vent kit that came with my Palmaro (Home Depot) WH is a Safe T (heat-fab) 4" stanless steel wall vent kit. I need to run mine through the roof. The diagrams I have suggest that a boot tee should be used if your are going to vent through the roof. It will be less than 5 feet from the top of the WH and the end of the vent. I spoke to a plumbing supply store in Houston that said their products were not compatiable with heat-feb. However, if my vent will be less than 5 feet, I can run the pipe stright up and put a rain cap on the end. Of course, using a fire stop, support, flashing and storm collar. Since he wasn't going to make a sale, I figured it was an honest answer. He said the pipe would be so short and its a forced vent that condisation would not be a problem.
Any thoughts?
It would sure save me time and money as the tee's from heat-fab are not cheap or easy to come by.
My distance is so short that if you exented the pipe it would be going out the roof instead of the wall any way.
Also, can you use a wall thimble on the celing of the landry room, as long as you also use a fire stop?