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 $219.47   9.358%   $240.008   $240.01   $240.00   $240.00  
  Purchase #2 $63.26   3.203%   $65.2862   $65.29   $65.30   $65.30  
  Purchase #3 $135.38   6.788%   $144.5696   $144.57   $144.55   $144.60  
  Purchase #4 $63.93   3.309%   $66.0454   $66.05   $66.05   $66.00  
  Purchase #5 $93.97   5.328%   $98.9767   $98.98   $99.00   $99.00  
  Purchase #6 $243.57   12.549%   $274.1356   $274.14   $274.15   $274.10  
  Purchase #7 $61.49   11.820%   $68.7581   $68.76   $68.75   $68.80  
  7 purchase totals = $881.07       $957.7796   $957.80   $957.80   $957.80