I pretty much bought my way out of the 10 & 25 tier. I bought 17 or 18 of the first 25. I haven't bought one since. Those first few tiers are tough just because it's a numbers game. On those tiers, you are right in that you might go with a larger niche or something trending. Price them cheap as possible just to get the sales and don't worry about making too much until you get tiered to 100 or 500.
Think of what's coming up and make some shirts on that. Going back to school, vacation shirts could still do good, fall sports like football & volleyball, Halloween in a month or so, etc.