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 $17.99   11.118%   $19.9901   $19.99   $20.00   $20.00  
  Purchase #2 $181.29   9.647%   $198.779   $198.78   $198.80   $198.80  
  Purchase #3 $79.81   4.582%   $83.4669   $83.47   $83.45   $83.50  
  Purchase #4 $194.83   7.532%   $209.5046   $209.50   $209.50   $209.50  
  Purchase #5 $199.27   9.684%   $218.5673   $218.57   $218.55   $218.60  
  Purchase #6 $230.30   4.545%   $240.7671   $240.77   $240.75   $240.80  
  Purchase #7 $12.25   3.810%   $12.7167   $12.72   $12.70   $12.70  
  7 purchase totals = $915.74       $983.7917   $983.80   $983.75   $983.90