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 $149.58   6.313%   $159.023   $159.02   $159.00   $159.00  
  Purchase #2 $181.43   4.216%   $189.0791   $189.08   $189.10   $189.10  
  Purchase #3 $114.32   11.522%   $127.492   $127.49   $127.50   $127.50  
  Purchase #4 $208.54   10.725%   $230.9059   $230.91   $230.90   $230.90  
  Purchase #5 $143.96   12.493%   $161.9449   $161.94   $161.95   $161.90  
  Purchase #6 $182.62   8.767%   $198.6303   $198.63   $198.65   $198.60  
  Purchase #7 $129.76   4.539%   $135.6498   $135.65   $135.65   $135.60  
  7 purchase totals = $1,110.21       $1,202.7250   $1,202.72   $1,202.75   $1,202.60