I read XML files that sometimes contain elements like
<stringValue>text
text</stringValue>
XmlReader returns
text\ntext
for such strings.
So, when I rewrite the source XML later using XmlWriter I don’t get the same strings (there is no 
 in them).
Should I worry about all this or it’s fine to allow string to be changed this way?
I would worry about it yes because your manipulating the data. This means if you do a round-trip to the XML document the text formatting wouldn’t be the same.
You would need to make sure on saving back out to XML persist the same formatting.