Depends on the Company you are flying with / for.
As an employed pilot, you follow the Company uniform policy, which as a rule of thumb is usually something along the lines of
1 Bar = Junior Line Pilot or Copilot
2 = Regular Line Pilot
3 = Senior Base Pilot or Check & train
4 = Chief Pilot, CFI etc.
If you are working through a full-time pilot program with one of the Uni-style operators, they may also have a uniform policy, say
None = GFPT
1 = PPL
2 = CPL
3 = MECIR
Instructional schools also use the bars to recognise instructor grades,
1 = Gr 3
2 = Gr 2
3 = Gr 1
4 = CFI
If you are employed by a company without a uniform or uniform policy, I would recommend either wearing neat civvies or a plain white shirt with a mind to fire and sun protection (ie long sleeves / pants / decent shoes/boots).
If you are a private pilot wear civvies (with the same caveat as above) or folks will think you a bit odd.....
regards.