Hi lillyjgc,
Planetary hours are one of the considerations; it doesn't meant that a chart is necessarily invalid if the hour and the ascendant don't match up or are not compatible by nature, triplicity, ruler, etc.. however when they *do* match up you can be sure (other considerations being ruled out) that the chart is 'tuned in' to your concern. It's just another way of confirming that the question you are asking is a 'good' one.
The moment you ask your question, your chart is frozen at that moment. So it doesn't matter if you go to a planetary hour calculator two days or two years later, the planetary hours for that day and that time will have remained the same. Just keep track of when you asked the question, or better, create your chart first, save it somewhere, and THEN go look up the planetary hour. That way you won't feel like you've 'lost' your question!
I'll include a link to the other astrological site I visit with too much regularity.
It's a table for 'compatibility' between ascendant rulers and planetary 'matches'. It's very helpful for deciding whether you have a match or not. But again, just because they don't match doesn't mean it's not a decent chart, as the other considerations will have a huge influence on it.
http://www.skyscript.co.uk/hourmatch.html
Best wishes,
AG