There’s a property appraiser in my area who is known as “[First name], true to the task at hand, [Last name]” in recognition of his/her ability to produce appraisals that always somehow seem to fit the needs of the entity commissioning/paying for the appraisal.
(e.g., tax assessments commissioned by cities/counties [and appraisals commissioned by sellers] always run high; condemnation appraisals commissioned by cities/counties/public utilities/highway-transportation officials [and appraisals commissioned by purchasers] always run low; and depending on the economy and funds available/required, appraisals commissioned by lenders and/or borrowers can go either way)
Although this appraiser and his/her appraisals are clearly inconsistent and subject to change, they "always remain true and faithful to the task at hand"
(i.e., “the current task that [he/she] is doing[;] that [task that he/she is] currently working on.” from Urban Dictionary).
Because of the possible positive connotation of being “true/faithful to the task at hand” (at least when the current task is a noble one), I think that this phrase/notion could be used successfully in your example (where the current task is the pleasing of a paying customer, and therefore perhaps not so noble) to subtly and jokingly express the notion that you’re after, perhaps as follows:
“[Don’t worry,] I’m [well-known as/proud to be known as] someone whose opinions and testimony always remain true and faithful to the task at hand.”