Depends on the altcoin. Don't buy something like DOGE or those shit other coins that some people just pump and dump. Best to keep just buying sure one like ETH, XMR or something like LTC. Some would look like good buys like that DOGE, since it has been pumping the past couple of days, but it is not really something that is legitimate.
Also, probably you should wait for the next bitcoin pump since it would drop the prices of alts and that is the best time to buy. A lot of alts are just extremely expensive already.