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 $171.82   10.335%   $189.5776   $189.58   $189.60   $189.60  
  Purchase #2 $175.83   9.179%   $191.9694   $191.97   $191.95   $192.00  
  Purchase #3 $9.29   4.725%   $9.729   $9.73   $9.75   $9.70  
  Purchase #4 $91.66   7.208%   $98.2669   $98.27   $98.25   $98.30  
  Purchase #5 $241.77   5.463%   $254.9779   $254.98   $255.00   $255.00  
  Purchase #6 $178.81   5.839%   $189.2507   $189.25   $189.25   $189.30  
  Purchase #7 $89.33   3.437%   $92.4003   $92.40   $92.40   $92.40  
  7 purchase totals = $958.51       $1,026.1718   $1,026.18   $1,026.20   $1,026.30