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 $6.85   7.350%   $7.3535   $7.35   $7.35   $7.40  
  Purchase #2 $96.44   8.511%   $104.648   $104.65   $104.65   $104.60  
  Purchase #3 $193.41   5.122%   $203.3165   $203.32   $203.30   $203.30  
  Purchase #4 $124.25   11.200%   $138.166   $138.17   $138.15   $138.20  
  Purchase #5 $138.79   12.325%   $155.8959   $155.90   $155.90   $155.90  
  Purchase #6 $225.42   13.579%   $256.0298   $256.03   $256.05   $256.00  
  Purchase #7 $114.63   7.805%   $123.5769   $123.58   $123.60   $123.60  
  7 purchase totals = $899.79       $988.9866   $989.00   $989.00   $989.00