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 $249.93   12.643%   $281.5286   $281.53   $281.55   $281.50  
  Purchase #2 $116.90   4.386%   $122.0272   $122.03   $122.05   $122.00  
  Purchase #3 $170.34   6.633%   $181.6387   $181.64   $181.65   $181.60  
  Purchase #4 $126.51   6.659%   $134.9343   $134.93   $134.95   $134.90  
  Purchase #5 $30.70   6.807%   $32.7897   $32.79   $32.80   $32.80  
  Purchase #6 $151.78   8.595%   $164.8255   $164.83   $164.85   $164.80  
  Purchase #7 $68.98   8.201%   $74.637   $74.64   $74.65   $74.60  
  7 purchase totals = $915.14       $992.3810   $992.39   $992.50   $992.20