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 $230.84   12.441%   $259.5588   $259.56   $259.55   $259.60  
  Purchase #2 $153.78   10.133%   $169.3625   $169.36   $169.35   $169.40  
  Purchase #3 $147.70   13.246%   $167.2643   $167.26   $167.25   $167.30  
  Purchase #4 $241.92   3.236%   $249.7485   $249.75   $249.75   $249.70  
  Purchase #5 $68.46   12.820%   $77.2366   $77.24   $77.25   $77.20  
  Purchase #6 $15.99   5.267%   $16.8322   $16.83   $16.85   $16.80  
  Purchase #7 $120.92   7.830%   $130.388   $130.39   $130.40   $130.40  
  7 purchase totals = $979.61       $1,070.3909   $1,070.39   $1,070.40   $1,070.40