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 $19.79   11.624%   $22.0904   $22.09   $22.10   $22.10  
  Purchase #2 $79.41   13.204%   $89.8953   $89.90   $89.90   $89.90  
  Purchase #3 $221.35   8.380%   $239.8991   $239.90   $239.90   $239.90  
  Purchase #4 $6.97   10.598%   $7.7087   $7.71   $7.70   $7.70  
  Purchase #5 $37.76   10.846%   $41.8554   $41.86   $41.85   $41.90  
  Purchase #6 $124.36   6.743%   $132.7456   $132.75   $132.75   $132.70  
  Purchase #7 $91.45   7.255%   $98.0847   $98.08   $98.10   $98.10  
  7 purchase totals = $581.09       $632.2792   $632.29   $632.30   $632.30