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 $58.65   13.287%   $66.4428   $66.44   $66.45   $66.40  
  Purchase #2 $204.76   3.222%   $211.3574   $211.36   $211.35   $211.40  
  Purchase #3 $157.51   10.461%   $173.9871   $173.99   $174.00   $174.00  
  Purchase #4 $100.38   6.751%   $107.1567   $107.16   $107.15   $107.20  
  Purchase #5 $16.83   11.146%   $18.7059   $18.71   $18.70   $18.70  
  Purchase #6 $243.50   12.726%   $274.4878   $274.49   $274.50   $274.50  
  Purchase #7 $16.80   9.315%   $18.3649   $18.36   $18.35   $18.40  
  7 purchase totals = $798.43       $870.5026   $870.51   $870.50   $870.60