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 $19.83   7.414%   $21.3002   $21.30   $21.30   $21.30  
  Purchase #2 $5.60   5.337%   $5.8989   $5.90   $5.90   $5.90  
  Purchase #3 $85.54   7.171%   $91.6741   $91.67   $91.65   $91.70  
  Purchase #4 $245.74   9.365%   $268.7536   $268.75   $268.75   $268.80  
  Purchase #5 $192.31   3.675%   $199.3774   $199.38   $199.40   $199.40  
  Purchase #6 $99.66   8.610%   $108.2407   $108.24   $108.25   $108.20  
  Purchase #7 $157.63   10.873%   $174.7691   $174.77   $174.75   $174.80  
  7 purchase totals = $806.31       $870.0140   $870.01   $870.00   $870.10