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 $16.69   8.461%   $18.1021   $18.10   $18.10   $18.10  
  Purchase #2 $138.48   8.776%   $150.633   $150.63   $150.65   $150.60  
  Purchase #3 $142.53   12.598%   $160.4859   $160.49   $160.50   $160.50  
  Purchase #4 $127.55   8.454%   $138.3331   $138.33   $138.35   $138.30  
  Purchase #5 $32.56   12.640%   $36.6756   $36.68   $36.70   $36.70  
  Purchase #6 $228.32   4.843%   $239.3775   $239.38   $239.40   $239.40  
  Purchase #7 $107.51   6.313%   $114.2971   $114.30   $114.30   $114.30  
  7 purchase totals = $793.64       $857.9043   $857.91   $858.00   $857.90