There will be substantial differences in statistics between these two characters. A level eight soldier / level one guardian will have, in most cases, 90 vitality, 44 Force points, nine feats, two Force powers, and somewhere around 12 skill points allocated. A level one soldier / level eight guardian, on the other hand, will have 90 vitality, 72 Force points, five feats, nine Force powers, and, again, somewhere around 12 skill points allocated. The first character will probably be somewhat more deadly in strict weapons-based combat, due to the extra Feats from his soldier levels, but the latter character will be more dangerous overall, due to the larger Force point reservoir and the greatly increased number of Force powers.
Obviously, if you intend to bring a character all the way from the Endar Spire to Dantooine without leveling up, you're going to have a hell of a hard time doing so, since you won't have more than 6--12 vitality at most. It's certainly possible, especially if you use your teammates to fight all of your battles in solo mode, and are willing to make many, many trips to the load game screen. There is a kind of sweet spot, however, that will probably make a bit more sense for most players. Soldier characters will find it easiest to reach level four before ceasing to level-up; this will give them a few feats under their belt, which will make it easier for their Jedi characters to deal damage in combat later on, since the Jedi typically gain fewer feats than do the regular character classes. For the same reason, and also to gain more vitality, the scout and scoundrel may want to reach level five before they stop leveling; they each gain one more feat this way, and the extra life will make the harder fights in the latter stages of Taris a bit easier to manage. Also, keep in mind that your skill progression will drop sharply once you become a Jedi; if you are relying on your player-character to pick locks or do other skill-related tasks, then you'll want to have a few more starting-class levels when you become a Jedi.
There is one other significant drawback to this method of leveling-up: while your character will likely be more powerful, your teammates will not gain quite as much experience when they're not in your party, if indeed they gain any at all. This bug is exacerbated if you choose to "save" levels earlier on: if, for instance, you cease leveling up from the very beginning of the game, your party members will usually lag behind your player-character by almost 30,000 experience. This means that they will be significantly underpowered for much of the mid-game, but by the time you reach the end of the adventure, this gap will be much less than a level, and so things will be a little less complicated. The experience gap is more severe if you try to save more levels, so if you wait until level four or five, your teammates will not lag as far behind.
Of course, you can feel free to proceed in Knights of the Old Republic however you see fit. If you do manage to survive Taris with a low-level character, however, your super-Jedi will make for a very fun playing experience, and will make your character more versatile in his or her use of Force powers as the game goes on. Indeed, this is the biggest difference between the normal method of leveling and this loophole-leveling; characters who exploit a loophole like this will gain more Force powers in the latter stages of the game. If you just want to chop stuff up with a lightsaber, this won't make a large difference in your playing style, but if you plan to become a Jedi consular, you can expect to be spoilt for choice when choosing your Force powers if you do manage to save a few levels before Dantooine.
Now, with that lengthy lemma out of the way, let's proceed to an explanation of the Jedi classes.