A good question. I think it depends on how badly you want someone. The reason why people get so salty is because they save hundreds of sq for a character and then never get that character. This is because of the drop rates. You have a 1% chance to get a 5-star in general, but if you divide that by the number of 5-stars there are in general that means you only have 0.017% chance by default, and a 0.7% chance when they're rateup.
My personal philosophy is to not save for any particular servant. I usually see a banner and spend 30-90 sq based on how I feel about, and then I take whatever I'm handed. The only two people I broke this rule for were Jalter (I spent real money to get her) and Skadi (I spent 400 sq but never got her. Or any 5-star, which is mathematically impossible).
So personally, I think if you don't feel strongly about anyone in the game, then just spend 30-60 sq on any banner that looks good.