Dear Miss Manners,
I’ve been invited to submit to some agents. Is it rude to submit to others as well as the ones I met?
Baffled in Buffalo
Dear Baffled:
Unless any agent has requested and been granted an exclusive, Agent Manners feels that there is no etiquette precluding multiple submissions. However, it is polite to inform the agents that others are also reviewing the materials. (Note: This is for requested partials and manuscripts; any agent who doesn’t assume these days that queries are going out far and wide is living under a rock.)
Though it isn’t spelled out in the question, Agent Manners presumes that the agents met were at a conference and the other agents would be approached via the query method, which resulted in requests for materials to be submitted. By all means, make the submissions. Just deal with everyone honestly and openly.
Et in arcaedia, ego. 
Dear Miss Manners,
Please remind your readers of the difference between multiple submissions (several pieces to one submitant at one time) and simultaneous submissions (one pieces to several submitants at one time). When speaking of short pieces (as opposed to novel queries), many more magazines are willing to look at multiple submissions than at simultaneous submissions.