The increase in insurance cost is for every $1000... so there shouldn't be an issue unless you're deciding between numbers below and above a break point. Always choose the amount at the high range of the break point... i.e. Insure it for $1000 for anything between $1-$1000 or $2000 for anything between $1001-$2000. Only complication comes when you mess with the $4000 number since I believe that is the cutoff where they start to ship by Registered Mail.
For the 1984 Topps Tiffany Mattingly/Strawberry, I'd just put $1000.
For the 1984 Topps Tiffany Mattingly/Strawberry, I'd just put $1000.