Oh boy...where to start? First, there are no strict rules about "sealing stone" - in fact, it's much like the fashion world - where there are few rules and everybody has on opinion. So saying 'travertine must be sealed' is really over-simplifying a question that has as many answers as it has questions. It may
be an easy "yes" but not a slam-dunk in my mind...
Second, know that with what we have from you so far, we can't give you an answer from 'here' - because we are not 'there' - we can't see what you've got and you haven't told us.
I can't even say for sure you're talking about your floor...
? are you? I guess you are - but I've seen travertine countertops so the question is
valid...then what finish do you already have on it? is it highly polished (like a mirror), or honed, like a satin pillow? can you see all the little holes in the stone, from pinhead sized to about the size of a pencil eraser? are they filled with something or not? See, I can make an assumption here but you know what they say about assumptions, don't you?
Most likely it is 'filled' (it wouldn't be sanitary in a kitchen unfilled)...and 'honed'. But then again, I am assuming your supplier was either a local tile store, or a big box store and that you live in an average house, not a $3 million-mansion...so you paid around $4-6 per sq ft?
OK once we've established what you have, we then ask what you want to do. You say you want to "seal it"...from what exactly? water? from dirt? from red wine stains? from scratches made by the dog? are you thinking a natural look or a topical sealer...something like putting a wax down on a wood floor? You mention "moderate traffic"... what? 2-adults-in-slippers or four-teenagers-and-two-dogs traffic? I'll assume a bit of both.
Your guy should have tested your travertine for absorption of liquids first and then given you his thoughts on sealing it. He is right in saying it is not complete sealing and that it will need to be resealed from time to time, but he is specifically referring to a topical sealer when he mentions grout and re-buffing. Perhaps he is unsure about what type of sealer you want? does he suggest an impregnator-sealer? which one? water-based or solvent-based? How many coats? how long between coats, if more than one...how does it work and what is it going to give you - and what is his liability in all that, if something happens...will he have to solve the problems? Is he capable?
So perhaps there was some "fluff" there, when harder answers should have been given. Maybe he's a tile setter and not a stone restoration expert, so he doesn't know...not his fault entirely. So maybe he wasn't the guy to ask...
But to leave travertine as is and re-hone it every couple of years is possible - but not as easy as buffing a waxed floor with a low-speed buffer. Guys that re-hone have specialized equipment and it's a process very similar to sanding a piece of rough wood to get a table-top.
I don't want to go on, already enough to think about as it is - but these are some of the areas that need addressing if we are to answer to your satisfaction the question you raise. If nothing else, I hope you see the size of the question. But
, if I assume you have filled, honed, absorbent travertine on a kitchen floor, with a moderate family and that you are looking for peace of mind and having told you that re-honing is something you might have to look at every few years, then I'd say seal it.
process may take you a few days, depending on the stone and the sealer...