ATC doesn’t really “read” flight plans at all. If the next sector is 8.33, they will hand you over to them, no matter what’s in your flightplan.
However, IFPS now no longer accepts flightplans without the “Y”, so you need to put it in either way.
IFR flight plans without Y are technically speaking not valid for the countries where it is mandated (Germany, France, etc.). I don’t believe the flight plan checker enforces it yet.
Just tried it with IFPS, flight plans without Y are accepted.
Legally you need to mark 8.33 Khz in the FPL when flying IFR through Germany as the rule is applicable since Jan 2014. There is a list of countries check the AIP for latest status:
All IFR/GAT flights or parts thereof operating in 8.33kHz airspace above FL195 within the IFPZ or below FL195 in the airspaces where 8.33 kHz is implemented are required to carry the 8.33kHz radio equipment, except where exemptions apply (as defined in the national AIP of the State concerned) and as specified by the terms of those exemptions.
Whenever an aircraft is equipped with the 8.33 kHz radios, the letter Y shall be inserted in Item 10: Equipment, of the filed flight plan.
achimba: There is surely 833 checking done on the IFPS server when routing (maybe not activated yet..?)
To my understanding this is based on Airspaces (Radar Sector configurations) loaded in the system which have certain attributes like 833 checking ….
I’ll turn it on 
and put Y in flight plan. I can always use other radio for 25 spacing and GNS for 8.33 if I’m bored with turnning GNS wheel.
… if I’m bored with turnning GNS wheel.
Flying IFR with a piston single, you will get maybe one frequency change every 20 or 30 minutes. So I assume you won’t get too bored. If you
would fly an SR71, I could understand your worries better 
I got a FP rejected last year, for not having the Y.
Don’t recall from where to where, however.
Knob twiddling is the worst aspect of the GNS430 and 8.33 doesn’t improve things. In some TMA departures you can go through several frequency changes in not many minutes. OK – if you know the route well you can pre-set and flip, that’ll give your 4 changes max. If you don’t know the route well, hand flying in turbulence and IMC, then this is a PITA. Like being given a direct to a waypoint not in your flightplan – the waypoint always seems to have 5 characters! Once again – in IMC and turbulence you can do without this.
Flying IFR with a piston single, you will get maybe one frequency change every 20 or 30 minutes. So I assume you won’t get too bored. If you
would fly an SR71, I could understand your worries better
Sometimes during departure it can be several frequencies that have to be changed – when I departed from LOWW I was on 5th frequency from turning main swhich on until I was taking off and in next 5 minutes I got two more changes. But you’re right – after that for an hour only 3 changes until landing.
I know we have had this here before but this IAOPA article clarifies the single 8.33 requirement for Germany
Funny (or not funny) how this has come such a short time after Germany “absolutely ruled” that two 8.33 radios are mandatory and some well known avionics shops refused to issue the notorious “IFR Certificate” over this.
