speaking from my own experience, welding aluminum on a steel table often works just fine.
But there are times when a part starts to chatter if not clamped down.
and there is risk of serious arc marks when that happens.
Especially if the table top is hot rolled steel.
An aluminum surface seems to conduct electricity better and less chance of that happening.
you can always just use a direct ground too.

I would encourage doing this. I'm planning on building a table using Vista Metals ATP-5 vertical cast aluminum tooling/jig plate. There have been quite a few different companies that have started to provide what they call "cast tooling plate". I'd make sure to find a place that is authorized to distribute the cast plate product (e.g. - Vista Metals MFG's ATP-5 and authorizes Midwest Steel & Aluminum as a distributor for their cast plate products, which comes with definite perks and assurances). If you do so, then details like flatness tolerances, surface RMS values, PVC protective covering on each side, etc. which appear in the product literature will be valid and assured. ATP-5 comes with face-milled surfaces on both sides, protective PVC covering, and a very narrow tolerance for flatness, thickness, and surface condition. ATP-5 is actually weldable and more corrosion resistant than the other alloys that are typical for this type of product, like MIC-6, etc., which is one of the big advantages to me. Also, it seems to be approximately the same price by volume as other standard plate alloys like 6061 and 5052.