Yes, it is perfectly OK to hold stocks in the US, and you do not ever need to transfer them out. If you are not a U.S. citizen or permanent resident, you are only subject to your home country's tax on investment income.
W8-BEN is not required for investment holdings (e.g. stocks, mutual funds), but is required for:
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Interest (including certain original issue discount (OID));
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Dividends;
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Rents;
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Royalties;
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Premiums;
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Annuities;
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Compensation for, or in expectation of, services performed;
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Substitute payments in a securities lending transaction; or
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Other fixed or determinable annual or periodical gains, profits, or income.
