From 3.5e SRD :
Spell Slots
The various character class tables show how many spells of each level a character can cast per day. These openings for daily spells are called spell slots. A spellcaster always has the option to fill a higher-level spell slot with a lower-level spell. A spellcaster who lacks a high enough ability score to cast spells that would otherwise be his or her due still gets the slots but must fill them with spells of lower level.
嗯,施法者 (不過這部份是放在 "準備法師法術" 裡) 可以用高等級slot準備低等級法術
術士和詩人就是沒提到... 不過反正 "沒" 說你不能這樣做!!
而且法師能,沒道理術士詩人不能 "浪費" 高等級slot去施展低等級法術
當然,除非是昇階過的法術,要不然都依原等級計算