As Gary said above flushing is a MUST DO if not the installation is incorrect and you boiler warranty maybe voided.
As your installer does he not install the boilers to 'MAUNUFACTURERS INSTRUCTIONS' or does HE just DECIDE to do it his way?
Remember under his DUTY OF CARE he is obliged to install this boiler correctly or he can LEGALLY be FORCED to come back and do it properly.
If he has told you he is RGI registered (and yes that would mean he would be listed on the site above) but isnt, then he is committing fraud.
The upshoit here is, your going to pay a few grand of your hard earned money to a guy you obviously can't trust and in a few years time when your new boiler starts to crapout on you due to poor installation I cannot see this guy helping you out. So why deal with him?
Paperwork you should receive from him after installation would be, ALL the boiler manuals (operation, service & installation manuals, they are not his to keep), any warning stickers or notices that come with the appliance, and most important, a CEDRTIFICATE OF CONFORMANCE (this ensure the installer has signed a legal document where he declares the installation has been done correctly and complies with all current gas installation regulations). Also if it was my house, I would insist that he COMPLETLEY fill out and sign the benchmark book for installation that will be found in the back of the boiler manual (this means he will have to detail that he DID flush out and add the correct inhibitors to the system (all installers are required to fill this out in the UK about time Irish customers insisted on it) and will help you should any warranty issues arrise where the manufacturer tries to blame it on the installer.
Sorry for the rant, but as Gary above, I do believe installations can and should be done to a high standard even if this means paying a little more as it ensure safe, efficient and long term operation of your entire heating system, which will hopefully do you for 15 or more years into the future!
ALWAYS INSIST ON RGI INSTALLERS (or RGII as it is now to be know from Jan 05 2009)