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 $250.68   13.573%   $284.7048   $284.70   $284.70   $284.70  
  Purchase #2 $37.53   9.805%   $41.2098   $41.21   $41.20   $41.20  
  Purchase #3 $240.47   12.696%   $271.0001   $271.00   $271.00   $271.00  
  Purchase #4 $55.38   8.534%   $60.1061   $60.11   $60.10   $60.10  
  Purchase #5 $226.81   4.377%   $236.7375   $236.74   $236.75   $236.70  
  Purchase #6 $239.48   6.362%   $254.7157   $254.72   $254.70   $254.70  
  Purchase #7 $117.66   7.446%   $126.421   $126.42   $126.40   $126.40  
  7 purchase totals = $1,168.01       $1,274.8950   $1,274.90   $1,274.85   $1,274.80