Let's say you get your tiles and you can spell "VALLEY". Is it legal to play "ALLEY" your first turn and then add the "V" your second turn? Assuming no double word score squares or anything, you would end up with 20 points over 2 turns instead of 12 in 1 turn. Would it really be worth it (obviously in this example it's not a very high-scoring word, but you get the idea)?
Yes, it's a perfectly legal scrabble move. Of course it assumes that nobody else makes a move in the meanwhile that prevents you doing this on your next turn (either places their own V or else puts another word down that doesn't leave you space to add the V). And whether the extra 8 points would be worth it would depend on what else you could have done instead (which you won't know until you draw those extra 5 files).