Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $9.59   13.401%   $10.8752   $10.88   $10.90   $10.90  
  Purchase #2 $6.28   9.587%   $6.8821   $6.88   $6.90   $6.90  
  Purchase #3 $158.47   13.630%   $180.0695   $180.07   $180.05   $180.10  
  Purchase #4 $242.91   3.718%   $251.9414   $251.94   $251.95   $251.90  
  Purchase #5 $93.93   5.556%   $99.1488   $99.15   $99.15   $99.10  
  Purchase #6 $203.97   7.401%   $219.0658   $219.07   $219.05   $219.10  
  Purchase #7 $197.82   11.305%   $220.1836   $220.18   $220.20   $220.20  
  7 purchase totals = $912.97       $988.1664   $988.17   $988.20   $988.20