The panels I have are rated for 1Kv operation above the surrounding aluminium frame, which I believe is fairly standard.
No problem in connecting a long string of panels in series to operate at high dc voltages.
If bolting the aluminium surround onto a steel frame, it's probably a good idea to have low voltage insulation to prevent galvanic corrosion. Many people do not bother and do not seem to have a problem. But if near the sea, its probably something to think about.
As Brad says, if the panels are very high, lightning could be an issue.
And if on a steel roof, always be aware of the possibility of electrocution.
A two pole isolating switch is usually a requirement for higher voltage systems.