Depending on what batteries you have, the discharge rates and settings are going to be a little different. A GP Battery can take a deeper discharge then an IB based battery. Typically, discharging cells to 0.90 volt (per cell - or 5.4 volts per 6-cell pack) is common. IN some race applications - and when done by an experienced racer, other numbers are used - however, for most off-road applications, 5.4 volts is a good target. Discharging at 1 Amp is going to take a very long time and offer very little advantage. A minimum 10-Amp discharge rate should be used... ideally, 20 Amp rates will offer the most benefit without overheating your cells.
While the discharger built into some chargers are going to work in getting your batteries down to the target 5.4 volts - there are much better dischargers available on the market if you are really looking for the best performance from your cells.
The whole purpose of discharging a battery is several-fold...
1) To test the quality of the cells in your pack - grading your cells based on the discharge time required to bring packs to a set voltage. Typically, cells are discharged at a rate equal to most racing applications - anywhere from 20 - 30 amps... however only very high-end chargers will allow this level of discharge and unless you are looking to grade the cells in your pack for matching purposes - discharging at high rates is really the only way to get real performance numbers. In most cases - a typically off-road racer is not going to need to do this step...
2) Reduce memory in the packs. NiMH batteries offer a great advantage over their predecessor... the NiCD battery in that they can provide much more voltage and run-time, and are a vast improvement over the memory problem that occurred with NiCD batteries, however, unless discharged completely before the next charge, a crystalline build-up occurs in the cells during the charging process. After time, this crystalline build-up leads to cells unable to hold the charge that they did when they were newer. This is main reason "pulse" dischargers were customized and marketed in the RC industry... to help break-down the crystallization formed in cells during the regular charging - running process. Basically, when a pack is fully discharged before charging, the process of crystallization is minimized. Better-yet, the use of a pulse discharger would further reduce crystallization in the cells. Both Muchmore Racing and Trinity make excellent dischargers that offer this feature.
3) Grading of the pack as a whole. By measuring voltage and charge-time (as well as mAh) you can get a pretty good idea of which of your batteries are going to perform their best. Starting all your batteries at 5.4 volts (6 cell NiMH batteries) , you will have a good base-line based on the charging performance of your packs. Although the track is always the best proving ground - if you have a battery that provides 25% more voltage and holds more mAh then any of your other batteries - money says that this is probably going to be your best pack.
While a built-in discharger may help get you to the ideal 5.4V starting point for your 6-cell battery packs, there is a BETTER option. With a vast majority of drivers moving to Lipo, you'll be able to find excellent deals on used Muchmore CTX-D's and Trinty Dyna-pulse discharge units. If you are looking to get the best performance out of your NiMH batteries, invest into one of these to ensure you are getting the best performance and longest lifespan from your cells.