When you "reclass" half of your level is stored as "internal level", which means that, although you can change a lvl10 mercenary to a lvl 1 myrmidon, it'll get experience as a lvl 6 unit, which means less experience per attack and kill. Lvl 6 vs lvl 1 isn't such a big impact though, but if you hit lvl 20, it'd mean that your unit would be counted as a lvl11 unit, which does add up after a while.
When you level up characters, they can get skills which can be carried even if you class change. For base classes, you get skills at lvl 1 and 10, so if you want to class change to another base class with a second seal, there's little reason to go beyond that.
Now, this is for reclassing with Second Seals. You can promote, class change to a stronger class, with the Master Seals. Now, the mechanics here work a little differently. A promoted class at lvl 1 counts as a lvl21 base class, doesn't matter when you used the promotion item - so, both using at lvl10 and lvl20 will result in a character that's considered by the game as lvl 21 (Continuing the earlier example, if the unit had 5 internal lvls due to using a second seal, then it'll actually be at lvl 26 after this promoted, doesn't matter when it was done). Some people still recommend using the Master Seal early just so you can get the extra abilities of the promoted class sooner (and also the skills, which are gotten at lvl 5 and 15), but that means you'll increase your internal level and lvl up slower after that.
You can also use the Second Seal to change to a promoted class directly to another one, but you'll need to hit at least lvl 10 on the promoted class to do that. If you aren't trying to get the lvl 15 skill of your initial promotion, then class changing at that point might be better. Of course, what I said earlier about internal lvls still apply here, and they'll increase quite a bit due to the promoted classes counting as at least lvl 21.