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 $221.57   12.201%   $248.6038   $248.60   $248.60   $248.60  
  Purchase #2 $43.49   10.159%   $47.9081   $47.91   $47.90   $47.90  
  Purchase #3 $246.93   4.329%   $257.6196   $257.62   $257.60   $257.60  
  Purchase #4 $48.91   7.569%   $52.612   $52.61   $52.60   $52.60  
  Purchase #5 $88.37   6.235%   $93.8799   $93.88   $93.90   $93.90  
  Purchase #6 $87.70   4.861%   $91.9631   $91.96   $91.95   $92.00  
  Purchase #7 $201.60   11.816%   $225.4211   $225.42   $225.40   $225.40  
  7 purchase totals = $938.57       $1,018.0076   $1,018.00   $1,017.95   $1,018.00