if you wait and finish piecing before dealing with the runs---you risk setting the dyes (by pressing--heat setting) so unless you are sure you will not do that you could wait- but if youi are pressing as you go you really need to (fix it) before continuing on- sorry-
reds, purples, and deep blues are the biggest culprits when it comes to this---and really should always at least be tested before cutting--just in case- even if you do not tend to ever pre-wash-