So long as the plug is installed in the tread belt, no problem. I've driven for years on self-installed tire plugs with no collateral damage. If the plug is on the sidewall or close to the edge of the tread belt, it's worth considering a new tire or a professional repair.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]