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 $204.59   12.573%   $230.3131   $230.31   $230.30   $230.30  
  Purchase #2 $232.97   10.729%   $257.9654   $257.97   $257.95   $258.00  
  Purchase #3 $47.76   5.395%   $50.3367   $50.34   $50.35   $50.30  
  Purchase #4 $212.53   3.209%   $219.3501   $219.35   $219.35   $219.40  
  Purchase #5 $125.34   12.141%   $140.5575   $140.56   $140.55   $140.60  
  Purchase #6 $76.33   3.542%   $79.0336   $79.03   $79.05   $79.00  
  Purchase #7 $35.35   11.458%   $39.4004   $39.40   $39.40   $39.40  
  7 purchase totals = $934.87       $1,016.9568   $1,016.96   $1,016.95   $1,017.00