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 $247.29   11.577%   $275.9188   $275.92   $275.90   $275.90  
  Purchase #2 $223.74   11.722%   $249.9668   $249.97   $249.95   $250.00  
  Purchase #3 $16.72   12.512%   $18.812   $18.81   $18.80   $18.80  
  Purchase #4 $105.79   7.290%   $113.5021   $113.50   $113.50   $113.50  
  Purchase #5 $163.60   10.727%   $181.1494   $181.15   $181.15   $181.10  
  Purchase #6 $77.76   10.566%   $85.9761   $85.98   $86.00   $86.00  
  Purchase #7 $66.85   3.503%   $69.1918   $69.19   $69.20   $69.20  
  7 purchase totals = $901.75       $994.5170   $994.52   $994.50   $994.50