I would never worry about it, but you should always give your chicks, and adult birds for that matter, a visual inspection every so often to look for signs of stress or illness.
Pasty butts usually occur because of the stress of transportation. It can also occur if the chicks are kept at an uncomfortable temperature, especially too cold. It can also occur because of too much fruit, dairy or other "non-feed" foods in their diet. Finally, coccidiosis can cause pasty butt, but that's minor compared to the coccidiosis itself. Chicks are at risk of coccidiosis infection between about 3 weeks and 15 weeks, so yours are entering that phase now. I am sure there can be other causes of pasty butt, but those are the chief ones.
But, again, it is not something to worry about. Just keep an eye out for general health issues from one day old on to...gulp...retirement.
I have never had a pasty butt except for the first day or two after I have acquired chicks, whether through the mail or at the feed store. If your chicks are not stressed or overdosing on non-feed foods, and if you keep them on the floor in clean litter (where they gradually build resistance to coccidiosis) you can stop worrying about it and just keep a calm watch over your flock for signs of any kind of illness.
UGCM