With Saitou, a lot of English speakers will assume the end of the name is pronounced like the end of the word too. That would be wrong.
With Saitoh, some might think the end of the name is pronounced like the 'o' in toffee. That would be wrong.
And with Saitō, if you don't know what the diacritical mark, a macron in this case, means, then you will be left unsure as to how to pronounce the word.
Note: to my undoubtedly warped way of thinking, it seems far better to leave the un-savvy reader unsure, rather than completely misapprehending, the pronunciation of a long vowel sound.
And presupposing any of these treatments of the long vowel sound is knowing the Japanese way to pronounce the vowel in the first place. If the name did not end with a long vowel sound, and was simply read as 'Saito', then one needs to know at the outset that the letter 'o' is pronounced in the same way as in the English word toe.
So when we append something to that vowel to indicate it is a long 'o' sound, whether ~ou, oh, or ō, then what that is signaling is that we utter the terminal 'o' sound for twice as long as otherwise.
Getting your long and short vowel sounds correct is absolutely essential to spoken Japanese if you want to get your meaning across. Without it, you can sound really funny, as these examples hopefully indicate:
This first one I found on the web elsewhere:
Others:“As a non-native Japanese speaker, I particularly pride myself on my pronunciation, but every now and then I slip up when it comes to long vowel sounds. Case in point: on [the JET Programme] at my junior high, some topic in the textbook led to a discussion about wind chimes, or fuurin. Unfortunately, I then announced to the class that I was a big fan of furin (adultery) instead…”
Ryokō (旅行), meaning travel, versus ryōkō (良好) meaning favorable.
Horyo (捕虜), meaning prisoner of war, versus hōryō (豊漁), meaning good catch (in fishing)
Shinju (真珠), meaning pearls, versus shinjū (心中), meaning double suicide
Kōbi (交尾), meaning copulation, versus kobi (媚び), meaning flattery.
Obāsan (おばあさん) meaning grandmother, versus obasan, (おばさん) meaning aunt.
Yūki (勇気), meaning courage, versus yuki (雪), meaning snow.
One can well imagine the embarrassing mix-ups and confusion in conversation that can occur from erring with the long and short vowel sounds. And that's just one pitfall....