"All fly rods are usually happy with a line weight up, or a line weight down."
Yes!
And some brands more one way or another - I have a Winston I like, which suits #5 and #6 lines, rated a #6 - I've had Sage rods rated #5, #6 and #7 (XP, TCR etc) which all suit heavier lines. This is about how a rod brand gets a sort of signature 'feel'. For example modern Hardy rods tend to be stiff, they might suit a line heavier especially if you are casting short,. eg fishing a small water.
So, for most normal fishing a DT line is not really a distance line - casting geeks may disagree. Similarly a sink tip is not an ideal distance line - even casting geeks cannot disagree
I'd suggest there is a fair chance the DT5 line I was playing with might be a little light or a little heavy depending on your taste and the type of fishing you are doing. It's worth experimenting with your fly rods, using a line light or a line heavy.
I read a warning online, on a well known forum, not this one, that using a line heavier or lighter than the maker's recommended line can break rods and or can void your warranty. Nonsense!
Magnus