If you can afford it and justify it for trip reasons, dry suit over wet suit anytime. Especially if you are going solo in challenging water temps or conditions, it's some peace of mind for you and those you left at home. (danger is still danger, don't let it change your judgement. It certainly doesn't make you invincible either.) Some conditions are just really challenging to dress and be comfortable in (for instance hot weather, frigid water) and nothing is going to feel right when you are paddling.
A few Alaskan trips I took w guides, they made you hop in the river or lake water the night before, just for the shock value...you'd know it was deadly cold and limb and brain numbing very quickly. Their point was always made, be prepared, have a plan, take the conditions seriously.
In the end, what Robin said is most sensible. Be careful, consider the conditions, know and respect your limits (and experience of your companions), have a backup & rescue plan and gear and think, think, think...