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 $234.67   6.832%   $250.7027   $250.70   $250.70   $250.70  
  Purchase #2 $72.26   10.194%   $79.6262   $79.63   $79.65   $79.60  
  Purchase #3 $115.26   10.743%   $127.6424   $127.64   $127.65   $127.60  
  Purchase #4 $236.50   11.607%   $263.9506   $263.95   $263.95   $264.00  
  Purchase #5 $25.28   5.576%   $26.6896   $26.69   $26.70   $26.70  
  Purchase #6 $115.83   13.385%   $131.3338   $131.33   $131.35   $131.30  
  Purchase #7 $178.77   12.788%   $201.6311   $201.63   $201.65   $201.60  
  7 purchase totals = $978.57       $1,081.5764   $1,081.57   $1,081.65   $1,081.50