Hi,
The NE555 timer chip supports strong output currents (TI specifies +/- 225mA) and therefore adding a parallel LED circuit is of no concern and in fact the preferred solution. It will work equally well with small signal LEDs as well as super-bright LEDs (which require higher currents but will be clearly seen even in sunshine). The only concern in that case is the load on the on-board 5V to 3.3V regulator in the Raspberry Pi to avoid overloading it (early Pi's had a linear regulator that only supported 50mA. now they have a switching regulator that will support 500mA).
However care must be taken when using outputs from other chips (such as low-power micro-controllers) that aren't designed to drive large loads. I designed a simple circuit that uses the drive current for the PTT transistor to illuminate a small signal LED. The original resistor is replaced with a LED and resistor in series. The value of the new resistor is lower to compensate for the voltage drop of the LED. The current through the LED is large enough to illuminate a small signal LED but also small enough to be within the design limits of the GPIO output and the transistor input. This is also an efficient design for battery operation since there is no increase in power consumption.
Thanks
Thomas
KK6FPP
