As a TMA radar controller, if I tell you to continue runway heading, then the heading is what I want. OK obstacle clearance may be an issue at some places, or to approach/deps radar if you're not above 1000' or so, but can't think of a situation where you'd be talking to me and so low as to be an issue.
And of course, I'd never dream of giving you a heading below the noise restriction altitude, would I?
Fair point though, and if you're unsure, ask!
Personally, I only say it if I want to stop you taking a SID turn, so +/-10 degrees, who cares, though obviously off parallels closing on other runway dep/go around could be fun.