Change of base currency does not mean that you are doing currency exchange. It changes the currency of your account balance calculations. That is it. You can either make an exchange on IDEAL or keep your money intact. When you do a trade in different currency, you are effectively borrowing it with your cash as collateral. It is up to you to choose a way to go.
A margin account is probably the best way to go. However if you don't wish to use leverage go to ACCOUNT and look at how much leverage ur using. is this something like 1 or <1, then you're fine. It will show you're using margin but you're not really using them.
So what is the best way to go here. My base is EUR and trade in USD only. I'm holding overnight as well so I'd like not to pay the borrowing rate. If taking an equal position (compared to EUR) of USD on IDEALPRO, would that help?
You need to check the interest rates paid (from IB to you) versus the interest rates charged (paid by you to IB). These are posted on their web site. You'll find that for most currency pairs you'll pay more in interest than you'll earn on the loan (if any interest is even paid). In a few cases you can play the carry trade if you want. For example, buy AUD with USD and you'll earn more in interest from your AUD positive balance than you'll be charged for your USD loan.