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 $232.27   10.353%   $256.3169   $256.32   $256.30   $256.30  
  Purchase #2 $22.28   3.685%   $23.101   $23.10   $23.10   $23.10  
  Purchase #3 $56.72   5.806%   $60.0132   $60.01   $60.00   $60.00  
  Purchase #4 $114.45   5.339%   $120.5605   $120.56   $120.55   $120.60  
  Purchase #5 $195.49   6.407%   $208.015   $208.02   $208.00   $208.00  
  Purchase #6 $62.91   7.758%   $67.7906   $67.79   $67.80   $67.80  
  Purchase #7 $226.73   7.559%   $243.8685   $243.87   $243.85   $243.90  
  7 purchase totals = $910.85       $979.6657   $979.67   $979.60   $979.70