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 $123.46   5.290%   $129.991   $129.99   $130.00   $130.00  
  Purchase #2 $230.38   6.808%   $246.0643   $246.06   $246.05   $246.10  
  Purchase #3 $85.85   5.734%   $90.7726   $90.77   $90.75   $90.80  
  Purchase #4 $132.49   8.339%   $143.5383   $143.54   $143.55   $143.50  
  Purchase #5 $106.80   12.861%   $120.5355   $120.54   $120.55   $120.50  
  Purchase #6 $20.37   5.793%   $21.55   $21.55   $21.55   $21.60  
  Purchase #7 $84.33   10.226%   $92.9536   $92.95   $92.95   $93.00  
  7 purchase totals = $783.68       $845.4053   $845.40   $845.40   $845.50