if it is raining hard, it is possible that water will sip in into your hubs, bottom bracket, headset, etc. This problem is specially true for lower-end group sets, but is not totally elliminated even on high-end groups.

You can always detect this by checking how smooth your wheel turns, etc...

If you detect abnormalities(higher resistance), you should lube this components. Bearings with no lubrication can heat-up and eat-away the casing, and vice versa.

Don't lube it using WD40 or other spray lube products, it will only dilute the grease(solid lubricant) and cause more damage.