In my chassan classes I was taught that it is important to have relations on the night a wife returns tahor from the mikvah. I do not know the source of this idea. My question is: May the couple choose to forgo relations on mikvah night for any reason, out of convenience?
It appears that yes, indeed, you can postpone relations if both spouses agree, but it is best avoided.
It is better not to postpone marital relations, but if it will not be comfortable for one or both of the couple, then it is permitted to postpone.
On the other side postponing shouldn't be taken lightly, see for instance this relating to the importance of the mitzvot post-tvila from yoatzot.org
One should make every effort to go to mikveh on appropriate night, even if it is inconvenient, so as not to delay the mitzvot of onah (marital relations) and peru urevu (procreation).
According to yoatzot.org, "[y]ou are not required to be intimate on mikveh night." The situation under discussion in that article is where the couple wants to abstain because they are angry at each other rather than simply out of convenience; however, the claim is stated without qualification.
There is an inyan of the klipos being misdavek to a woman who goes to the mikva and does not subsequently have relations with her husband.
According to this it would be better to delay going to the mikva to a night when they do intend to have relations subsequently.
Bearing in mind that if she sees spotting etc. you will be soser the shiva nekiim.