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 $115.40   12.712%   $130.0696   $130.07   $130.05   $130.10  
  Purchase #2 $133.32   12.435%   $149.8983   $149.90   $149.90   $149.90  
  Purchase #3 $138.69   9.107%   $151.3205   $151.32   $151.30   $151.30  
  Purchase #4 $12.67   3.132%   $13.0668   $13.07   $13.05   $13.10  
  Purchase #5 $225.26   10.132%   $248.0833   $248.08   $248.10   $248.10  
  Purchase #6 $152.82   7.786%   $164.7186   $164.72   $164.70   $164.70  
  Purchase #7 $226.94   6.554%   $241.8136   $241.81   $241.80   $241.80  
  7 purchase totals = $1,005.10       $1,098.9707   $1,098.97   $1,098.90   $1,099.00