The price of Bitcoin and Ethereum will vary from day to day. You are fine to buy those coins at any price as long as it's not moving around too quickly at the time you are making your purchase. Ideally, the price will have been fairly stable for several hours or even more than a day. Then once you have bought your Bitcoin or Ethereum, you can then turn around and buy your altcoin fairly quickly, and you shouldn't suffer any ill effects from Bitcoin or Ethereum price fluctuations.
The scenario you want to avoid is that situation where you are buying Bitcoin during a time the price is dropping rapidly. In that case your purchasing power for altcoins can quickly erode, as the altcoin prices will tend to increase relative to Bitcoin to keep pace with their normal US Dollar value. It's not nearly so bad if you buy Bitcoin during a time when its price is rapidly increasing, because then your altcoin purchasing power goes up as the altcoin price relative to Bitcoin will tend to drop. However, there are times when both the price of Bitcoin and the price of altcoins go up rapidly at the same time, and that can make trading very challenging.
As long as you are buying altcoins, you want to look for the good ones. What makes a good altcoin?
1. It's been around for a while (new coins could turn out to be good but you don't know that yet).
2. Healthy community
3. More than one person on the development team
4. Coin has a plan beyond mere speculation
5. Coin has some value-added infrastructure
6. Coin has incentive for people to hold onto it (good proof of stake returns are the most common incentive)
7. Substantial amounts of the coin are kept off the open market (i.e., by people holding it or it being locked up in some way)
There are a few good altcoins out there which are currently undervalued. One in particular is
Diamond Coin (DMD). Check it out; do your homework. I believe you will find that DMD meets all the above requirements.