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 $14.63   13.759%   $16.6429   $16.64   $16.65   $16.60  
  Purchase #2 $116.90   5.121%   $122.8864   $122.89   $122.90   $122.90  
  Purchase #3 $156.90   3.581%   $162.5186   $162.52   $162.50   $162.50  
  Purchase #4 $120.77   6.783%   $128.9618   $128.96   $128.95   $129.00  
  Purchase #5 $223.53   10.204%   $246.339   $246.34   $246.35   $246.30  
  Purchase #6 $172.25   11.346%   $191.7935   $191.79   $191.80   $191.80  
  Purchase #7 $118.39   9.721%   $129.8987   $129.90   $129.90   $129.90  
  7 purchase totals = $923.37       $999.0409   $999.04   $999.05   $999.00