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 $247.85   11.535%   $276.4395   $276.44   $276.45   $276.40  
  Purchase #2 $146.27   5.436%   $154.2212   $154.22   $154.20   $154.20  
  Purchase #3 $189.40   9.155%   $206.7396   $206.74   $206.75   $206.70  
  Purchase #4 $41.31   4.579%   $43.2016   $43.20   $43.20   $43.20  
  Purchase #5 $39.72   10.727%   $43.9808   $43.98   $44.00   $44.00  
  Purchase #6 $134.73   13.431%   $152.8256   $152.83   $152.85   $152.80  
  Purchase #7 $130.46   9.602%   $142.9868   $142.99   $143.00   $143.00  
  7 purchase totals = $929.74       $1,020.3951   $1,020.40   $1,020.45   $1,020.30