It depends on which format film you talk about and which enlarger you use. I can only speak for myself and my enlargers, but what goes for my system may not work for yours.
I use Leitz Valoy II, Focomats 1C an 2C and a Durst L1000.
The Valoy II and 1C have a condenser that presses down on the negative: no further glass involved and wonderful engineering of the glassless negative holder, all remains 100% flat. For the Valoy II Leitz made socalled Anti Newton condensers (standard in the later grey and lightgrey versions) and for the 1C there's a anti newton glass that slips on the condenser.
The 2C has diferent negative holders. Most common is the one with two glasses, the top one anti newton. This enlarger does 135 to 6X9. With this holder everything is flat but you are cleaning four sides of the glasses. For 6X6 there are metal holders made by Kienzle. For the 2C I have never seen any metal holder for larger than 6X6.
Less common is the 135mm negative holder, which is precisely what you are talking about: plain glass below and a metal holder on top. It works perfectly and reduces the cleaning to only the bottom glass.
Over the years I have gotten extra metal holders for 135 and 6X6, that I have all filed out a little, also the ones made by Kienzle. These give me more black border around image which I then mask off with easels.
The Durst L1000 has a holder that has either glass, or metal masks. I haven't had this enlarger long enough to experiment with glassless.
I may be wrong but sometimes, with the 2C, I have felt Anti Newton glass does do something to the image. Let's say it evens it out more, or makes it a little less in contrast. This certainly does not go for all negatives. So sometimes I change to glassless holders for that reason. Or sometimes, for 135mm, I switch from 1C to 2C with the Newton glass in the holder because that solves a problem.