Hmm... I had a friend with this problem and the HV made her come in for weigh-ins every week. It almost made her crazy. Her DD was BF, but she admitted that she'd been dieting while BFing (low carbing) which i don't think helped much! Anyway, I digress...
I don't know enough about this, but personally I would postpone a week if she has been ill - you wouldn't want to take her to the GP's if she's been ill.
Oh FFS some bloody HVs (and I can be sarky because I am one - not an irritating one I'll have you know just one with common sense). The centile line most babies are on at birth bears no resemblance to the one they will be on at a year. Has your HV never heard of "catch down". Just as some babies who are a lower birth weight race up the centiles so others who are on the upper centiles race down a bit until they find the one they wish to follow (has mental image of baby around the 9th centile going "hmm this one looks comfy).
Your HV is wrong - personally I only ever worry if they drop through three centiles in 6 weeks or less (rare) the last baby I saw do that had a heart problem and had caused concerns from birth and her Mum was the one saying "what's the matter with my baby - the heart problem was not picked up just because her weight dropped through the centiles. There were other concerns
What is the HV going to suggest you do anyway? Your DD is the size she's meant to be, hitting her milestones, content, healthy and happy. Why can't the HV be saying "what a fantastic job you've done with her"? Too simple when she can over medicalise everything and create concern.
Frankly if you aren't concerned I'd postpone for a lot longer than a week and not take her back until her 8 month check if then. In any event I certainly wouldn't take her in on Tuesday if she's been ill. It will be a lot of stress and tell you nothing of any actual use.