300 feet is the acceptable error standard for Mode C equipment ... i.e. if your reported and Mode C altitudes differ by 300 or more feet, ATC will ask you to "stop altitude squawk" ... so I've always used 300 feet as the "trouble mark". En-route, deviations of a mile or more are usually necessary before they'll be noticed, especially by Center radar. On an instrument approach, half-scale deflection is a good rule of thumb. I don't think these vary too much from the northeast to the other parts of the country.
Having said that, you can "get away with" the foregoing most of the time. However, your goal should be to fly as accurately as possible at all times. When I was instructing full-time, I might have a private pilot who could hold altitude within 80 feet ... many CFIs would be happy with that, since the PTS requires 100 feet. I would encourage my students not to be happy with 80, but strive for 50, then 20, etc. Personally, I strive for 20 foot altitude, three degrees heading, four knots airspeed, and one dot needle deflection as my standards ... way tighter than commercial or ATP requirements. Do I do that well regularly? Of course not ... but I keep encouraging myself to be more precise, and not to tolerate small deviations. If you can hold 9980 feet perfectly, why not climb 20 feet and still have it nailed?
File IFR and get out in the system. With the complexity of the airspace and the volume of traffic in the Northeast, I almost never operate VFR up there. Listen up on the radio, comply promptly with ATC requests (safety first, of course), and learn standard/preferred routings for your area. I believe you're in the PHL area ... IFR departures are always going to go out over four fixes from there ... DITCH for east and northeast bound traffic, Modena (MXE) for west and southwest bound traffic, Pottstown (PTW) for northwest and north bound traffic, or Woodstown (OOD) then Salisbury (SBY) for southbound flights. Unlike other parts of the country where "cleared direct" to some distant fix is fairly common, in the northeast you'll get a lot more airway clearances, and be expected to fly them. If you get shortcuts, it's typically to a VOR or intersection just a couple fixes down your route, rather than "direct Cleveland" from PTW. Also study the STARs for PHL and vicinity ... some are listed as "turbojet only" but you'll be given similar routings. The BUNTS1 is for turbine airplanes, but coming in from the west/northwest piston aircraft will be assigned "HAR V210 BUNTS" which is exactly the same thing. Same thing with the Cedar Lake (VCN7) arrival ... they'll rattle it off in airways over VCN then OOD, but it is the same routing as the STAR, essentially.
How's that for a hundred dollar answer to a ten cent question?
Good luck!
FL270