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 $128.97   4.660%   $134.98   $134.98   $135.00   $135.00  
  Purchase #2 $63.25   4.771%   $66.2677   $66.27   $66.25   $66.30  
  Purchase #3 $155.41   3.354%   $160.6225   $160.62   $160.60   $160.60  
  Purchase #4 $117.92   5.549%   $124.4634   $124.46   $124.45   $124.50  
  Purchase #5 $41.58   6.729%   $44.3779   $44.38   $44.40   $44.40  
  Purchase #6 $203.85   11.298%   $226.881   $226.88   $226.90   $226.90  
  Purchase #7 $205.95   11.532%   $229.7002   $229.70   $229.70   $229.70  
  7 purchase totals = $916.93       $987.2927   $987.29   $987.30   $987.40