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 $27.45   10.633%   $30.3688   $30.37   $30.35   $30.40  
  Purchase #2 $30.46   8.483%   $33.0439   $33.04   $33.05   $33.00  
  Purchase #3 $57.35   6.414%   $61.0284   $61.03   $61.05   $61.00  
  Purchase #4 $114.41   8.755%   $124.4266   $124.43   $124.45   $124.40  
  Purchase #5 $208.75   10.699%   $231.0842   $231.08   $231.10   $231.10  
  Purchase #6 $131.69   5.494%   $138.925   $138.93   $138.95   $138.90  
  Purchase #7 $17.76   5.394%   $18.718   $18.72   $18.70   $18.70  
  7 purchase totals = $587.87       $637.5949   $637.60   $637.65   $637.50