Answer
Not exactly offensive. One may sit in a comfortable place or any available space, but there is a certain consideration to keep in mind. If you sit in a place that is meant for spiritual practice, it creates a different atmosphere. Even a small corner of a room with a table and a picture of Krishna, or any room where a picture of Krishna is placed, can have a different effect on the mind and heart.
In the beginning, when the heart is still becoming purified, chanting in such a place can be especially helpful. Chanting itself is not dependent on external circumstances. The holy name acts regardless of where you chant, when you chant, or how much you chant. Still, in the beginning it is advisable to create a favorable atmosphere.
For example, if you chant in a noisy place, it becomes difficult to hear what you are chanting because the noise acts as a distraction. But with practice, when the mind becomes fixed on chanting and hearing, then even in a noisy environment you will be able to concentrate only on the holy name without your attention being diverted by external sounds.
Therefore, in the beginning one should practice chanting in a favorable environment — a sacred place, a holy place, a clean place, and a relatively quiet place. It may not always be completely silent, but at least quiet enough that you can hear your chanting without much distraction.
So, it is not an offense to sit on the bed and chant. Certainly not. However, a bed is generally meant for sleeping and resting. If you chant there, the tendency may be to relax too much or even fall asleep. One may not remain as attentive or serious as when sitting properly in another place meant for practice.
That is the idea. It is not an offense, but creating a suitable atmosphere in the beginning helps one make quicker progress by chanting and hearing nicely, with minimum distractions, diversions, and obstacles.
