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 $84.31   4.650%   $88.2304   $88.23   $88.25   $88.20  
  Purchase #2 $144.48   6.475%   $153.8351   $153.84   $153.85   $153.80  
  Purchase #3 $127.33   10.214%   $140.3355   $140.34   $140.35   $140.30  
  Purchase #4 $60.63   6.812%   $64.7601   $64.76   $64.75   $64.80  
  Purchase #5 $91.64   5.154%   $96.3631   $96.36   $96.35   $96.40  
  Purchase #6 $181.91   5.769%   $192.4044   $192.40   $192.40   $192.40  
  Purchase #7 $233.74   12.322%   $262.5414   $262.54   $262.55   $262.50  
  7 purchase totals = $924.04       $998.4700   $998.47   $998.50   $998.40