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 $250.84   7.756%   $270.2952   $270.30   $270.30   $270.30  
  Purchase #2 $142.73   13.439%   $161.9115   $161.91   $161.90   $161.90  
  Purchase #3 $118.92   11.736%   $132.8765   $132.88   $132.90   $132.90  
  Purchase #4 $172.99   11.494%   $192.8735   $192.87   $192.85   $192.90  
  Purchase #5 $183.80   6.726%   $196.1624   $196.16   $196.15   $196.20  
  Purchase #6 $88.61   8.586%   $96.2181   $96.22   $96.20   $96.20  
  Purchase #7 $197.38   11.236%   $219.5576   $219.56   $219.55   $219.60  
  7 purchase totals = $1,155.27       $1,269.8948   $1,269.90   $1,269.85   $1,270.00