@Roqua:
You can gain as much xp as you want and you are never forced to level up, no matter how much xp you have gained. However, when you press that "+" button, you will advance to the highest level your xp can grant you. So yes, you can save every single point of xp you gain and you won't lose anything, but when you decide to advance you can't raise just to level X if you are eligible to level X+1.
Don't worry, unless you'll be grinding, you will rarely find yourself in a situation where you are eligible to e.g. +5 level advance (except maybe for the beginning when leveling goes quite quickly). That's because of the level scaling - the guys you meet at a certain level simply won't give you enoug xp. The requirements on leveling up grow fast enough, thus you always need some capable enemies who give you some serious amount of xp. I mean if you are lvl 5 and you defeat a band of suckers, you will gain enough to move significantly towards lvl 6, but not enough to make a serious progress towards higher levels. For that you would have to level up, therefore encounter tougher enemies who will give you more xp etc.
To your question - yes, you can stay at some level and just raise the skills, but remember that wizardry takes your level into account for many things - critical hits, spell and magical weapon effects (used either by you or against you), and of course with each level you gain precious hit points, stamina, mana and character attributes that lead to power skills (or what they're called) once you reach 100 in the corresponding attribute. Plus, although the game is level scaled, each area has its minimum and maximum level of enemies, so you could have a hard time in certain locations.
If you play a solo character, it's quite crucial not to level up too quickly, or you'll be dead before you'll realize it in the monastery
. If you have a full party though, I wouldn't worry about leveling at all, and as soon as you'll feel comfortable in the swamp I'm sure it'll be ok till the end.