The problem with deciding if Tannehill is a reach at #6 is twofold:
1. Has Shanahan seen enough of him, in his evaluation, to feel confident he can succeed in his system. If we assume that is a yes, then you have to consider...
2. Indianapolis takes Luck and Cleveland takes RG3; how far back can we go to ensure we still get him? Miami picks either 8th or 9th, depending on the outcome of the coin flip with Carolina. Carolina is not going to pick a QB, but Miami is. If we trade back to 10 or beyond, Tannehill is availabe at #8 or #9 and Miami will take him. So the only one we can trade back with comfortably is Jacksonville at #7. Jacksonville has no need to trade up 1 spot and does not need a QB.
So, taking Tannehill at #6 is not a reach if the Shanahans have determined from their evaluation of him that he is, or potentially is, the answer to their problem at QB. Last year's draft showed that Shanahan will not reach for a QB if he believes he is not worth it and can fix a hole elsewhere. However, after the failed Rex/Beck experiment, he may be inclined to take Tannehill and look for O-line/Receiver starters/playmakers with his 2nd, 3rd, and 2 4th round picks.