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 $99.57   6.300%   $105.8429   $105.84   $105.85   $105.80  
  Purchase #2 $21.27   4.457%   $22.218   $22.22   $22.20   $22.20  
  Purchase #3 $165.34   12.361%   $185.7777   $185.78   $185.80   $185.80  
  Purchase #4 $136.46   8.406%   $147.9308   $147.93   $147.95   $147.90  
  Purchase #5 $152.83   7.151%   $163.7589   $163.76   $163.75   $163.80  
  Purchase #6 $9.60   11.834%   $10.7361   $10.74   $10.75   $10.70  
  Purchase #7 $9.98   3.646%   $10.3439   $10.34   $10.35   $10.30  
  7 purchase totals = $595.05       $646.6083   $646.61   $646.65   $646.50