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 $121.92   13.192%   $138.0037   $138.00   $138.00   $138.00  
  Purchase #2 $99.79   9.536%   $109.306   $109.31   $109.30   $109.30  
  Purchase #3 $115.98   8.388%   $125.7084   $125.71   $125.70   $125.70  
  Purchase #4 $114.38   13.645%   $129.9872   $129.99   $130.00   $130.00  
  Purchase #5 $120.80   7.850%   $130.2828   $130.28   $130.30   $130.30  
  Purchase #6 $34.61   11.797%   $38.6929   $38.69   $38.70   $38.70  
  Purchase #7 $166.88   11.193%   $185.5589   $185.56   $185.55   $185.60  
  7 purchase totals = $774.36       $857.5399   $857.54   $857.55   $857.60