I would also pay attention to how much time you spend in the USA each year in total, one or two US flights a month together with an annual holiday could blip the radar for you having a significant presence there. Remember the US government is trillions of dollars in debt and will latch onto anything they think will get them some revenue. The IRS will send out tax returns on a "spray and pray" basis hoping some people will complete them and pay up. Even if you're not liable, the accountants and lawyers fees to prove this will likely exceed the tax due if you were liable.