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 $248.67   4.130%   $258.9401   $258.94   $258.95   $258.90  
  Purchase #2 $61.48   5.452%   $64.8319   $64.83   $64.85   $64.80  
  Purchase #3 $79.28   6.268%   $84.2493   $84.25   $84.25   $84.20  
  Purchase #4 $183.94   11.589%   $205.2568   $205.26   $205.25   $205.30  
  Purchase #5 $66.33   13.173%   $75.0677   $75.07   $75.05   $75.10  
  Purchase #6 $208.74   3.550%   $216.1503   $216.15   $216.15   $216.20  
  Purchase #7 $105.67   9.629%   $115.845   $115.85   $115.85   $115.80  
  7 purchase totals = $954.11       $1,020.3411   $1,020.35   $1,020.35   $1,020.30