Great question with a mixed answer! The reason some people recommend this is that a tent is more likely to leak the first time it gets wet. This is due to the needle holes made when sewing the tent together. When new tents get wet, the cotton shrinks around the thread and seals itself. So setting your new tent up and wetting it down could be a good idea. HOWEVER, keep reading for why it might not be the best idea.
When setting up your tent to hose it down, you could end up with a shrinkage problem unless you set it up VERY well. When we say very well we mean the same as you would set it up for elk camp. This means you must use EVERY rope and stake, make sure the ropes are TIGHT, and be certain the stakes are SECURE (this might be a challenge on soft backyard lawns). Under these “real hunting world” conditions, your tent would seal itself and retain its size and shape.
We don’t tell our customers to “pre-shrink” their tents as most set it up properly at camp and never have a problem.
Note: On internal frame tents, the fit will “float” around a bit. Think of a pair of jeans coming out of the dryer and you think you cant button them; by the end of the day they are loose. Similarly, your wall tent will fit the frame differently at times.