Hope this helps a bit. I'm 65, and I've used computers since 1983 (and before, technically). My son is a network administrator for a large company. Historically, many optimizers were poorly programmed, and "fix/repair" utilities did, in fact, often remove registry bits and "shared files" that were important to a particular app, causing problems. Most such fixers and optimizers in the last 5 years have been MUCH better at what they claim, but there are so many out there, it's like buying RAM: so many different speeds, kinds, pins, etc., it's not only confusing, you can (and I have done so) really mess up a computer accidentally.
So #1, do your research online to find reviews by real users (not just the company's site) and reviews by pro publications. It pays to do your homework.
#2, no optimizer or fixer is 100% perfect. Each has its foibles, and flaws. That's why research (similar computer, similar software, similar experience level) can significantly reduce your fears and the likelihood of problems.
#3, your pro is speaking from his experience, but what is his training and real world experience? He my be very savvy, or may be relying on old history. Unless you ask (and hope he doesn't take offense at a perfectly proper question), you don't know. Ok.
I use Norton 360 because they are highly rated, have reduced their resource use a lot, and they have a relatively easy interface to tell you what's going on. But 360, unlike Internet Antivirus, has several useful utilities that Norton has even historically been lauded for. Their disk optimizer, for example is significantly faster and more efficient than Windows. They have a registry cleaner that has not (in 4 years) given me one single error or crash.
I have tried Uniblue, but they keep pushing some other utilities that I had problems with. However, their free Registry Cleaner has performed excellently also, and if Norton didn't already have one, I'd likely upgrade from free to the fee one. I find no conflicts with Malwarebytes antivirus (PC Mag, Laptop, etc., recommended as a backup), and Secunia PSI (free or fee versions) does an excellent job on my Windows AND non-Windows upgrades and patches.
Since moving to Windows 7 (highly recommended, but if you go from XP to 7, have your pro help you), I have had 2" blue screens of death", one from a leftover file from Trend Micro PC-cillin that was hidden (and I still can't remove it for unknown reason) that Norton fixed, and the second one recently from a still unknown cause that seems to have fixed itself. With Vista, I had more than 20, with XP, I had about the same over a 5-year period. So I like 7, I like Norton 360 (if it's available in England), Secunia PSI, and Uniblue's Registry Cleaner. They seem to work well on my 3 computers (2 laptops and a desktop, all Windows 7 now). Again, hope that's useful to you.