Should I use a Cable Amplifier ?
Cable amplifiers are designed to boost antenna or cable TV signals. When a cable signal comes into a house, it is generally strong enough for 2-3 devices. If you have more than that (including everything that is connected to a coax cable, like cable modems, TiVo, ReplayTV, DVRs, cable boxes), you may need a cable amplifier.
When the cable comes into the house, it starts getting split using "splitters". Every time it is split with a 2-way splitter, it incurs 3.5db loss on each output leg. You can also think of this in terms of percentages, 100% signal in, 50% signal out on both legs. A 4-way splitter will cause 7db loss on each output (or 25% of the signal remains on each leg).
Cable modems use different frequencies than TV, and require a pretty strong signal with fairly unrestricted path back to the cable company. This is why cable companies will often put a separate line for cable modems, and/or they may often be split from TVs with a "tap" (a.k.a. directional coupler). Taps are a type of 2-way splitter that retains most of the signal on one output (0-1db, used for the modem), while it incurs most of the loss on the other (6-7db, used for TVs).
Cable modems will generally work with a splitter or a bi-directional amplifier. Depending on the signal strength, a bi-directional amplifier may improve or degrade your cable modem performance. The best advice here is, if it isn't broke, don't try to break it. If you are not experiencing issues, and/or you are not sure what your signal strength is, you do not need an amplifier for it.
Cable amplifiers not only amplify the signal, but also any noise on the line, and introduce some return loss. Bi-directional amplifiers only pass the signal back, they do not amplify the return signal - they even introduce some return loss (ustream signal loss typically about 1-2db). In most cases, it is very important for cable modems that the return signal is clear and unrestricted, that is why cable amplifiers may often degrade the performance of the cable modem. The exception being weak incoming signals, which is best handled by the cable company itself. Many modern cable modems have a web admin/stats page that shows the signal levels, http://192.168.100.1 for Motorola/Arris, for example.
To summarize, there are some situations where a quality bi-directional cable amp can help, but, before you decide on using one with a cable modem make sure to read its signal levels and make an educated decision. More often, you should split the cable modem with a tap, and put the cable amplifier only on the TV side after the cable modem. Only use cable amplifiers when experiencing issues with more than a couple of TVs present.
See also: What cable modem signal levels are considered good ?