As an LFS owner myself, I would say more precisely, "Support a good LFS". There are some awful ones around that don't deserve your custom.
My advice would be to never just make your decision based on price. This goes for livestock and dry goods. Having a knowledgeable LFS that can give you concise and accurate support on the equipment and choice of dry goods that meet your requirements is worth a premium. Clean well cared for livestock is often a symptom of the owner knowing about each of the animals individual needs.
Ask questions, a good store should be both open and detailed about there operating procedures.
Never forget, your LFS may well be your only emergency option if something goes wrong. Very rarely will you be able to get delivery from online stores the same day or within hours. This includes crucial items such as heating equipment, saltwater or RODI.
With regards stocking of brands. Unless you are a large store with a big turnover, you can`t just go out and buy every product, just to have it sat on shelves. Also most big brands require retailers to buy in minimum order quantities, this can be very expensive if you sell that brand only occasionally, or are a small store like myself. However for the vast majority of items I can supply though pre order, I can offer my customers price match from major online retailers.
Most of all your LFS should be your access to professional advice and support. If it is not, then search around for one that is.
I have on countless occasions had visitors to my store asking for advice about a product they have purchased online, or even sometimes from another store. Quite frankly I find this rather rude and very disheartening.
I run my store because I love the hobby and love talking to fellow reefers, I don't see how that is a disadvantage. I was very disappointed by the general standard of LFS, especially those trying to sell everything, Freshwater, Saltwater, Reptiles and everything in between. Without even a basic knowledge of the animals identification , never minds its requirements.
I am semi retired after been relatively successful in other business ventures. To be honest I think running an LFS as a serious business model just doesn't add up, far to many negative variables weighted against a minimum gain. Supporting your good LFS is not just to boost their profit margin , it can in many cases be a critical lifeline. Use it or it will be gone when you need it most.