Merlin - isn't that a bit broad and strong on the tap water commentary? There are places in the US - NYC being a notable one - where the tap water is superb. New York's water is so pure, it's close to RO quality.
I am now living in my 10th different residence while growing orchids (out of 18 altogether), have used tap water in all of them, and they did fine. Granted, that was before chloramines, which is something you definitely don't want to give them. It is better to check with your water authority than to rely on broad recommendations.
Back to the original question - as "King" said, your orchids will need to be fed no matter what the water supply. However, the fertilizer you use with tap water might not be the best in pure water:
1) Tap water typically contains calcium and magnesium carbonates, rainwater does not, and both calcium and magnesium are important nutrients.
3) Tap water is also buffered by those dissolved minerals, so adding a fertilizer to them is not an issue. Rain water, containing no significant quantities of anything, is unbuffered, so if you use the same fertilizer in it, the pH will plummet and be dangerous for the plants. You either have to adjust the pH back up to something in the 5.5-6.5 range, or get a fertilizer intended for use in pure water.
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