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 $167.85   10.116%   $184.8297   $184.83   $184.85   $184.80  
  Purchase #2 $15.76   10.667%   $17.4411   $17.44   $17.45   $17.40  
  Purchase #3 $211.43   4.372%   $220.6737   $220.67   $220.65   $220.70  
  Purchase #4 $16.95   11.650%   $18.9247   $18.92   $18.90   $18.90  
  Purchase #5 $120.70   10.705%   $133.6209   $133.62   $133.60   $133.60  
  Purchase #6 $176.44   9.836%   $193.7946   $193.79   $193.80   $193.80  
  Purchase #7 $231.51   7.596%   $249.0955   $249.10   $249.10   $249.10  
  7 purchase totals = $940.64       $1,018.3802   $1,018.37   $1,018.35   $1,018.30