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 $34.49   11.257%   $38.3725   $38.37   $38.35   $38.40  
  Purchase #2 $113.30   3.734%   $117.5306   $117.53   $117.55   $117.50  
  Purchase #3 $91.25   10.106%   $100.4717   $100.47   $100.45   $100.50  
  Purchase #4 $247.42   6.164%   $262.671   $262.67   $262.65   $262.70  
  Purchase #5 $204.75   13.322%   $232.0268   $232.03   $232.05   $232.00  
  Purchase #6 $148.55   3.772%   $154.1533   $154.15   $154.15   $154.20  
  Purchase #7 $14.42   6.438%   $15.3484   $15.35   $15.35   $15.30  
  7 purchase totals = $854.18       $920.5743   $920.57   $920.55   $920.60