The score of a word is the tile total multiplied by the word length and then any bonuses added, so one extra letter can make quite a difference to the value of a word. Whether it's worth moving and using a blank depends on the situation! In your example, the word "welder" is worth 72 points and "welders" (with a blank "s") is worth 84 points. Are 12 points worth 2 swaps? Up to you! If you make longer words you will get more gold tiles. They come in at a more or less regular pace - the more tiles you use, the more multipliers you will get. But you will also get more hot and broken tiles!