When do certain TEs become draft values?

Mostly my question is in the title. I usually will wait on a TE until the later rounds as I'm more interested in the WR/ RB values that fall through the cracks in rounds 3-7. But when do the top end TEs (Gronk, Kelce, Olson) become too good a value to pass up? Gronk in the thirties (3rd round in 12 team leagues), Kelce in the forties (4th round in 12 team leagues)? I'm just curious as to where people stand on this one.

TL/DR - At what point in the draft do the top TEs become too good a value to pass up drafting?