While an ultrasonic cleaner would be nice (and who doesn't want a new tool?), it shouldn't be necessary unless you have been using India ink or iron gall ink in the pen.
It is helpful to have a converter that you can use to draw water through the nib and feed, and then push the water back out. Flushing the pen half a dozen times with lukewarm (but NOT HOT) water normally should do the job.
If ink has dried in the feed, then you may need to soak the pen overnight. Plain water is usually good enough, but adding one drop of dishwashing detergent often helps. If you use detergent, you should flush the pen several times the next day to remove any trace of the detergent.
The usual issue is that ink has dried inside the feed, but there is also a concern that the ink could have stained the pen. There are a few inks that are notorious for their ability to stain pen parts - Noodler's Baystate Blue is probably the most infamous and will stain both plastic and metal parts. I happen to like BSB - it is a beautiful bright blue. But my experience is that the stain from Baystate Blue usually won't be touched by water, even with detergent. Adding a few drops of hypochlorite bleach usually clears it pretty quickly. If you flush the pen out thoroughly, the residual stain won't affect the next refill of the pen, so you can change colors without clearing the stain completely. But for obvious reasons, using BSB in a really expensive pen would probably not be wise.
