Toroids side cover magnetic induction
A uniformly wound toroid has no external magnetic field, regardless of the core permeability.

In a non-uniformly wound one, the external field is reduced by the core permability.

Given that most commercial toroidal transformers are wound relatively uniformly (for all sorts of good practical and financial reasons), and that the core permeability is generally in the 1000s, the external field of a toroidal transformer can be neglected, at least for gross worries like reducing the transformer efficiency or heating the cover. Even for fine worries, like inducing signal voltages in the inputs of amplifiers, a commercial toroid is usually good enough. 

A toroidal is an order of magnitude or two better than an EI or a C-core power transformer on external field, as the latter types tend to have some small air-gap where the parts of the core meet, making for a non-uniform core. A toroid cannot help but be gapless, however well or badly assembled.