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 $34.97   12.211%   $39.2402   $39.24   $39.25   $39.20  
  Purchase #2 $197.25   10.716%   $218.3873   $218.39   $218.40   $218.40  
  Purchase #3 $219.46   13.301%   $248.6504   $248.65   $248.65   $248.70  
  Purchase #4 $148.37   12.105%   $166.3302   $166.33   $166.35   $166.30  
  Purchase #5 $101.39   4.711%   $106.1665   $106.17   $106.15   $106.20  
  Purchase #6 $164.79   10.705%   $182.4308   $182.43   $182.45   $182.40  
  Purchase #7 $48.80   5.263%   $51.3683   $51.37   $51.35   $51.40  
  7 purchase totals = $915.03       $1,012.5737   $1,012.58   $1,012.60   $1,012.60