Removing metal tabs on Lutron dimmer switch in ganged box
If the dimmer will fit without touching any part of the other switch it's OK and advisable to leave the tabs in place since removing them will decrease the capacity of the dimmer @Harper thanks. Personally, I'd remove them if you're installing a few low wattage bulbs now just in case you add a larger switch in the future and maybe forget about the removable tabs on the dimmer. Then you'd have to come back here and ask how to make the switch fit in the box. :-)