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 $134.32   3.433%   $138.9312   $138.93   $138.95   $138.90  
  Purchase #2 $9.66   8.723%   $10.5026   $10.50   $10.50   $10.50  
  Purchase #3 $243.82   9.825%   $267.7753   $267.78   $267.80   $267.80  
  Purchase #4 $159.83   8.730%   $173.7832   $173.78   $173.80   $173.80  
  Purchase #5 $126.90   5.224%   $133.5293   $133.53   $133.55   $133.50  
  Purchase #6 $20.30   6.610%   $21.6418   $21.64   $21.65   $21.60  
  Purchase #7 $245.33   3.241%   $253.2811   $253.28   $253.30   $253.30  
  7 purchase totals = $940.16       $999.4445   $999.44   $999.55   $999.40