Spells cast cast out of higher level slots

[Falcii]

Can I cast fireseeds (a 6th level spell) out of 7th 8th and 9th level slots? Like it'd be cast as a 6th level spell, I'm not asking for free heighten spell, but for my current build I don't really want alot of my higher level slots.

[Fizban]

Yes, you can always prepare (or spontaneously cast) lower level spells with your higher level slots. In fact that's all you can do with them if your casting stat isn't high enough for the high level spells, but you can do it voluntarily as well. One of the more annoying problems with the Neverwinter Nights games is that they can't handle this, forcing you to waste feats on metamagic in order to fine tune your loadout.

[Necroticplague]

Can I cast fireseeds (a 6th level spell) out of 7th 8th and 9th level slots? Like it'd be cast as a 6th level spell, I'm not asking for free heighten spell, but for my current build I don't really want alot of my higher level slots.

Yes, you definitely can.

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.

[Psyren]

Note that you lose nothing by Heightening it, so if you have that feat you might as well - especially since this spell carries a saving throw, so Heighten allows you to reap the benefit of casting it out of the higher slot(s) by actually making it calculate as a higher-level spell.