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 $239.88   8.354%   $259.9196   $259.92   $259.90   $259.90  
  Purchase #2 $21.91   6.317%   $23.2941   $23.29   $23.30   $23.30  
  Purchase #3 $61.67   8.783%   $67.0865   $67.09   $67.10   $67.10  
  Purchase #4 $129.81   13.529%   $147.372   $147.37   $147.35   $147.40  
  Purchase #5 $243.96   8.189%   $263.9379   $263.94   $263.95   $263.90  
  Purchase #6 $183.31   12.645%   $206.4895   $206.49   $206.50   $206.50  
  Purchase #7 $135.32   13.184%   $153.1606   $153.16   $153.15   $153.20  
  7 purchase totals = $1,015.86       $1,121.2602   $1,121.26   $1,121.25   $1,121.30