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 $28.56   7.830%   $30.7962   $30.80   $30.80   $30.80  
  Purchase #2 $240.56   5.601%   $254.0338   $254.03   $254.05   $254.00  
  Purchase #3 $10.92   6.313%   $11.6094   $11.61   $11.60   $11.60  
  Purchase #4 $150.30   3.557%   $155.6462   $155.65   $155.65   $155.60  
  Purchase #5 $132.25   5.677%   $139.7578   $139.76   $139.75   $139.80  
  Purchase #6 $168.29   6.667%   $179.5099   $179.51   $179.50   $179.50  
  Purchase #7 $128.62   8.551%   $139.6183   $139.62   $139.60   $139.60  
  7 purchase totals = $859.50       $910.9716   $910.98   $910.95   $910.90