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 $75.27   12.476%   $84.6607   $84.66   $84.65   $84.70  
  Purchase #2 $245.47   12.293%   $275.6456   $275.65   $275.65   $275.60  
  Purchase #3 $208.58   11.155%   $231.8471   $231.85   $231.85   $231.80  
  Purchase #4 $79.51   8.224%   $86.0489   $86.05   $86.05   $86.00  
  Purchase #5 $68.25   8.717%   $74.1994   $74.20   $74.20   $74.20  
  Purchase #6 $35.99   11.136%   $39.9978   $40.00   $40.00   $40.00  
  Purchase #7 $174.83   8.450%   $189.6031   $189.60   $189.60   $189.60  
  7 purchase totals = $887.90       $982.0026   $982.01   $982.00   $981.90