Cheap But Brilliantly Effective upgrades

I'd start with a quarter, once round (valve-to-valve as mentioned above). The check tension/change in tension, check it's still true in relation to your brake blocks* (just tighten them up a bit using the barrel adjuster). Take it from there...

*if it's the front wheel, or you're into contingency.

and what if it's not still true? then i'm fucked, i'm sweating, i'm on my way to the LBS