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 $81.40   9.562%   $89.1835   $89.18   $89.20   $89.20  
  Purchase #2 $173.96   8.153%   $188.143   $188.14   $188.15   $188.10  
  Purchase #3 $145.95   4.123%   $151.9675   $151.97   $151.95   $152.00  
  Purchase #4 $114.64   4.338%   $119.6131   $119.61   $119.60   $119.60  
  Purchase #5 $141.63   12.377%   $159.1595   $159.16   $159.15   $159.20  
  Purchase #6 $159.58   12.846%   $180.0796   $180.08   $180.10   $180.10  
  Purchase #7 $19.88   8.708%   $21.6112   $21.61   $21.60   $21.60  
  7 purchase totals = $837.04       $909.7574   $909.75   $909.75   $909.80