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 $220.49   8.378%   $238.9627   $238.96   $238.95   $239.00  
  Purchase #2 $179.41   11.812%   $200.6019   $200.60   $200.60   $200.60  
  Purchase #3 $32.61   12.521%   $36.6931   $36.69   $36.70   $36.70  
  Purchase #4 $187.29   9.347%   $204.796   $204.80   $204.80   $204.80  
  Purchase #5 $201.80   7.276%   $216.483   $216.48   $216.50   $216.50  
  Purchase #6 $55.79   8.225%   $60.3787   $60.38   $60.40   $60.40  
  Purchase #7 $127.99   6.386%   $136.1634   $136.16   $136.15   $136.20  
  7 purchase totals = $1,005.38       $1,094.0788   $1,094.07   $1,094.10   $1,094.20