Sure, Sylvanas and Tirion are probably the best ones to Entomb, regardless of the situation. The question is, should you Entomb a lower value card to avoid losing the board? In some cases it's worth it.
I agree. And you really need to know the deck the opponent is playing to make that decision. Today I played a fatigue rogue. I noticed a bit too late which deck he played (or else I would have played Thoughtsteal earlier to try to get Gang Up). As soon as he played Deathlord, I Entomb'd the card. We both hit fatigue the same time and the game ended in a draw.
Is it worth to entomb a piloted shredder? Probably not... But how do you deal with those pesky T4/T6 shredders as a priest? Death lord doesn't cut it. Do you velen's chosen your northshire?
Yesterday i Entombed a Flamewaker. It was my opponent's second one and i already had dealt with Antonidas. A Shredder should ideally be traded by your opponent into your taunt, but it depends on the situation. There's no definite answer for that.
My first game with Entomb I had board control vs a paladin (Auchenai Soulpriest, Injured Blademaster + another minion), and he coined out Sylvanas on turn 5. This would have been a disaster pre LoE, but I had Entomb ready and effortless removed the threat. I won that game.
Remember, never see a card in vacuum. Example: Sylvanas Windrunner a tempo card which in most cases is a 2-for-1 card. You either burn at least 2 cards (silence/spell/minion) to deal with her and her deathrattle, or you burn 1 card (spell/minion) and let the deathrattle take another minion. Since Priest seldom has cheap tokens or silence, you will almost always lose 2 valuable cards. Entomb makes Sylvanas Windrunner a 1-for-1 card, which is makes the opponent lose the tempo advantage (which is equivalent to you gaining tempo).
Another way to see Entomb is a very flexible tech card. How do you deal with Sylvanas Windrunner? Silence and SWD. Tirion Fordring? Silence and SWD, or SWD and Harrison Jones. In both cases you need an extra tech card (Silence, Harrison Jones), which takes away a spot for other more flexible Priest card. Priest is master of combinations or bad card draws, but with Entomb you don't need to rely on specific tech cards, it can be used on most situations.
And as a final note (as someone has already said in this thread), with Entomb you don't need any late game minions. Against aggro, you will most likely not have enough mana to play Ysera, and Priest decks usually has no problems vs aggressive deck. Against control, Entomb is disgustingly good, because you will go to fatigue and you will use your opponents biggest minion against him. Mid-Range decks are Entomb's biggest weakness, but that's why you have Lightbomb.
Remember, never see a card in vacuum. Example: Sylvanas Windrunner a tempo card which in most cases is a 2-for-1 card. You either burn at least 2 cards (silence/spell/minion) to deal with her and her deathrattle, or you burn 1 card (spell/minion) and let the deathrattle take another minion. Since Priest seldom has cheap tokens or silence, you will almost always lose 2 valuable cards. Entomb makes Sylvanas Windrunner a 1-for-1 card, which is makes the opponent lose the tempo advantage (which is equivalent to you gaining tempo).
Another way to see Entomb is a very flexible tech card. How do you deal with Sylvanas Windrunner? Silence and SWD. Tirion Fordring? Silence and SWD, or SWD and Harrison Jones. In both cases you need an extra tech card (Silence, Harrison Jones), which takes away a spot for other more flexible Priest card. Priest is master of combinations or bad card draws, but with Entomb you don't need to rely on specific tech cards, it can be used on most situations.
And as a final note (as someone has already said in this thread), with Entomb you don't need any late game minions. Against aggro, you will most likely not have enough mana to play Ysera, and Priest decks usually has no problems vs aggressive deck. Against control, Entomb is disgustingly good, because you will go to fatigue and you will use your opponents biggest minion against him. Mid-Range decks are Entomb's biggest weakness, but that's why you have Lightbomb.