is it ok to reposition my Monstera deliciosa to stop it growing sideways?

Yes, if you can find a support for your Split-Leaf Philodendron (Monstera deliciosa) (like a moss pole or tree branch, etc.) it will grow in a more upward position as it does in nature, where it likes to climb trees.

Mine grows sideways in a pot too. It also has no support. But i find it is those roots that seem to be pulling it in one direction as they anchor themselves into the ground. I have a trellis. It's still waiting for me to put it up.

Yes, its okay to re-position the rootball in the pot so the plant is in a more upright position. However, because this is not a naturally upright plant, it is a matter of time before it starts leaning one way or the other. You can either let it vine or trail naturally or you can use some support system mentioned previously or you can simply keep it pruned back so that is doesn't lean or wander too far astray.