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 $34.82   12.594%   $39.2052   $39.21   $39.20   $39.20  
  Purchase #2 $240.61   4.598%   $251.6732   $251.67   $251.65   $251.70  
  Purchase #3 $207.33   8.387%   $224.7188   $224.72   $224.70   $224.70  
  Purchase #4 $91.83   4.154%   $95.6446   $95.64   $95.65   $95.60  
  Purchase #5 $140.54   7.455%   $151.0173   $151.02   $151.00   $151.00  
  Purchase #6 $232.25   3.373%   $240.0838   $240.08   $240.10   $240.10  
  Purchase #7 $240.78   9.491%   $263.6324   $263.63   $263.65   $263.60  
  7 purchase totals = $1,188.16       $1,265.9753   $1,265.97   $1,265.95   $1,265.90