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 $177.38   6.546%   $188.9913   $188.99   $189.00   $189.00  
  Purchase #2 $235.42   9.752%   $258.3782   $258.38   $258.40   $258.40  
  Purchase #3 $70.36   12.584%   $79.2141   $79.21   $79.20   $79.20  
  Purchase #4 $129.26   11.483%   $144.1029   $144.10   $144.10   $144.10  
  Purchase #5 $7.77   3.602%   $8.0499   $8.05   $8.05   $8.00  
  Purchase #6 $118.51   6.435%   $126.1361   $126.14   $126.15   $126.10  
  Purchase #7 $160.34   13.145%   $181.4167   $181.42   $181.40   $181.40  
  7 purchase totals = $899.04       $986.2892   $986.29   $986.30   $986.20