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 $116.77   4.338%   $121.8355   $121.84   $121.85   $121.80  
  Purchase #2 $73.90   13.768%   $84.0746   $84.07   $84.05   $84.10  
  Purchase #3 $27.35   7.502%   $29.4018   $29.40   $29.40   $29.40  
  Purchase #4 $10.26   9.823%   $11.2678   $11.27   $11.25   $11.30  
  Purchase #5 $147.28   7.535%   $158.3775   $158.38   $158.40   $158.40  
  Purchase #6 $110.66   13.220%   $125.2893   $125.29   $125.30   $125.30  
  Purchase #7 $7.62   8.819%   $8.292   $8.29   $8.30   $8.30  
  7 purchase totals = $493.84       $538.5385   $538.54   $538.55   $538.60