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 $203.76   6.517%   $217.039   $217.04   $217.05   $217.00  
  Purchase #2 $21.86   6.604%   $23.3036   $23.30   $23.30   $23.30  
  Purchase #3 $244.89   12.265%   $274.9258   $274.93   $274.95   $274.90  
  Purchase #4 $49.78   13.120%   $56.3111   $56.31   $56.30   $56.30  
  Purchase #5 $221.83   3.683%   $230   $230.00   $230.00   $230.00  
  Purchase #6 $47.39   7.546%   $50.966   $50.97   $50.95   $51.00  
  Purchase #7 $248.79   12.141%   $278.9956   $279.00   $279.00   $279.00  
  7 purchase totals = $1,038.30       $1,131.5411   $1,131.55   $1,131.55   $1,131.50