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 $25.39   13.680%   $28.8634   $28.86   $28.85   $28.90  
  Purchase #2 $158.66   9.787%   $174.1881   $174.19   $174.20   $174.20  
  Purchase #3 $80.41   10.405%   $88.7767   $88.78   $88.80   $88.80  
  Purchase #4 $35.91   8.238%   $38.8683   $38.87   $38.85   $38.90  
  Purchase #5 $80.37   6.451%   $85.5547   $85.55   $85.55   $85.60  
  Purchase #6 $9.67   3.370%   $9.9959   $10.00   $10.00   $10.00  
  Purchase #7 $76.94   6.329%   $81.8095   $81.81   $81.80   $81.80  
  7 purchase totals = $467.35       $508.0566   $508.06   $508.05   $508.20