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 $29.64   3.832%   $30.7758   $30.78   $30.80   $30.80  
  Purchase #2 $35.29   3.447%   $36.5064   $36.51   $36.50   $36.50  
  Purchase #3 $201.58   3.411%   $208.4559   $208.46   $208.45   $208.50  
  Purchase #4 $116.33   3.276%   $120.141   $120.14   $120.15   $120.10  
  Purchase #5 $32.37   4.418%   $33.8001   $33.80   $33.80   $33.80  
  Purchase #6 $239.83   6.192%   $254.6803   $254.68   $254.70   $254.70  
  Purchase #7 $122.85   10.328%   $135.5379   $135.54   $135.55   $135.50  
  7 purchase totals = $777.89       $819.8974   $819.91   $819.95   $819.90