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 $24.76   3.183%   $25.5481   $25.55   $25.55   $25.50  
  Purchase #2 $250.93   12.630%   $282.6225   $282.62   $282.60   $282.60  
  Purchase #3 $119.28   3.406%   $123.3427   $123.34   $123.35   $123.30  
  Purchase #4 $167.38   7.474%   $179.89   $179.89   $179.90   $179.90  
  Purchase #5 $23.98   13.669%   $27.2578   $27.26   $27.25   $27.30  
  Purchase #6 $172.87   12.107%   $193.7994   $193.80   $193.80   $193.80  
  Purchase #7 $5.69   5.441%   $5.9996   $6.00   $6.00   $6.00  
  7 purchase totals = $764.89       $838.4601   $838.46   $838.45   $838.40