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 $27.41   12.318%   $30.7864   $30.79   $30.80   $30.80  
  Purchase #2 $17.25   7.334%   $18.5151   $18.52   $18.50   $18.50  
  Purchase #3 $207.85   12.109%   $233.0186   $233.02   $233.00   $233.00  
  Purchase #4 $175.77   11.551%   $196.0732   $196.07   $196.05   $196.10  
  Purchase #5 $170.62   12.268%   $191.5517   $191.55   $191.55   $191.60  
  Purchase #6 $197.97   11.816%   $221.3621   $221.36   $221.35   $221.40  
  Purchase #7 $7.96   10.814%   $8.8208   $8.82   $8.80   $8.80  
  7 purchase totals = $804.83       $900.1279   $900.13   $900.05   $900.20