If it's Mr Lowden, general opinion is that 12s are usually best for standard tuning. Lighter, and they don't give enough oomph to drive the top; heavier, and they put more pull on the bridge which damps the top.

The issue is how much pull the strings exert on the bridge. If you *always* play in a particular tuning, then you might want strings to suit that specific tuning - so if your top string is always detuned to D, you might want a thicker top string to compensate. For most situations though, regular strings sound fine.

Graham.