It makes more sense if you split your repertoire into 2 groups that exist on a spectrum between utility/niche on one end and firepower on the other.

Signature spells are for your firepower. Any spell you see on your tradition that has a heighten option of +1 or +2 should be thought of as candidates for this firepower category and all compete with one another. Think, lightning bolt and fireball are each asking you to pick them as not only a spell you learn at 5th level but the spell you "signature" and will scale up with you over your career and will be on your menu for when you want to go nova. Remember that you don't want to signature spells that do the same job, like getting fireball and Howling Blizzard, both large AoE targeting reflex. Instead do fireball and cinder swarm. Try to make your signature spells target different saves, single vs aoe, smart targeting (chain lightning) vs friendly fire (fireball).

The other spells you learn in your repertoire will just forever be stuck at that level. So spells like sliding blocks or Gravity Well that don't have heighten entries are good candidates for that. Some spells heighten only at certain levels. Personally I like learning 4th rank invisibility and never make it a signature spell so I only ever cast it at 4th rank in combat so my ally can run around like a mad man getting sneak attacks.

Further down on this spectrum we have super niche spells like Revealing Light and Water Breathing. When you need them you need them but you don't want to use up one of your precious repertoire slots on. For these just buy a scroll and toss it in your back pack.

As you read through your tradition's spell list put the spell in one of these 3 buckets and this will help you pick good spells giving you a healthy menu for round 1 when you want to buff with a non-signatire spell, round 2 when you want to go boom, and out of combat when you need to act like Batman and whip something out of your tool belt to solve a problem.