Shielding causes capacitance. Capacitance can form a filter which can filter out highs. Depends on the application.
Best bet is to try it and listen.
ToN gave the main way it could affect tone.
You can look up the formula for factors that affect capacitance. Among them are area of the plates forming the capacitor, and the distance between the plates. If the area of the plates gets larger, capacitance goes up, and if the plates are placed closer together, capaciance goes up.
It's a bigger concerned with shielded cables, where the insulation between the shield and the center conductor forms a capacitor, and the "plates" (the 2 conductors) are fairly close together. But shielding in guitars is usually a matter of shielding the control and pickup cavities. The area of the shielding could be huge, but the distance between the shield and the rest of the circuitry is huge by comparison to a shielded cable.
I think it's a non-issue. People realized what I described, but didn't put numbers to it, look at where the "hidden capacitor is connected in the circuit, or realize that the resulting capacitance is likely in the fraction-of-a-pF range. If it shunted any signal, it would shunt extreme high RF energy, and that's good.