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 $28.94   6.646%   $30.8634   $30.86   $30.85   $30.90  
  Purchase #2 $83.67   3.200%   $86.3474   $86.35   $86.35   $86.30  
  Purchase #3 $23.90   8.221%   $25.8648   $25.86   $25.85   $25.90  
  Purchase #4 $166.57   7.537%   $179.1244   $179.12   $179.10   $179.10  
  Purchase #5 $35.98   6.338%   $38.2604   $38.26   $38.25   $38.30  
  Purchase #6 $49.25   12.319%   $55.3171   $55.32   $55.30   $55.30  
  Purchase #7 $188.61   13.289%   $213.6744   $213.67   $213.65   $213.70  
  7 purchase totals = $576.92       $629.4519   $629.44   $629.35   $629.50