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 $246.98   10.563%   $273.0685   $273.07   $273.05   $273.10  
  Purchase #2 $52.36   13.298%   $59.3228   $59.32   $59.30   $59.30  
  Purchase #3 $160.98   8.215%   $174.2045   $174.20   $174.20   $174.20  
  Purchase #4 $98.38   4.710%   $103.0137   $103.01   $103.00   $103.00  
  Purchase #5 $12.84   10.126%   $14.1402   $14.14   $14.15   $14.10  
  Purchase #6 $195.40   3.331%   $201.9088   $201.91   $201.90   $201.90  
  Purchase #7 $144.53   4.719%   $151.3504   $151.35   $151.35   $151.40  
  7 purchase totals = $911.47       $977.0089   $977.00   $976.95   $977.00