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 $102.31   3.167%   $105.5502   $105.55   $105.55   $105.60  
  Purchase #2 $81.57   7.871%   $87.9904   $87.99   $88.00   $88.00  
  Purchase #3 $122.78   6.575%   $130.8528   $130.85   $130.85   $130.90  
  Purchase #4 $215.80   13.301%   $244.5036   $244.50   $244.50   $244.50  
  Purchase #5 $189.27   4.619%   $198.0124   $198.01   $198.00   $198.00  
  Purchase #6 $8.45   11.411%   $9.4142   $9.41   $9.40   $9.40  
  Purchase #7 $201.40   6.589%   $214.6702   $214.67   $214.65   $214.70  
  7 purchase totals = $921.58       $990.9938   $990.98   $990.95   $991.10