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 $24.77   11.116%   $27.5234   $27.52   $27.50   $27.50  
  Purchase #2 $47.58   4.295%   $49.6236   $49.62   $49.60   $49.60  
  Purchase #3 $131.34   12.146%   $147.2926   $147.29   $147.30   $147.30  
  Purchase #4 $220.57   7.870%   $237.9289   $237.93   $237.95   $237.90  
  Purchase #5 $94.60   12.541%   $106.4638   $106.46   $106.45   $106.50  
  Purchase #6 $203.26   7.380%   $218.2606   $218.26   $218.25   $218.30  
  Purchase #7 $84.64   11.268%   $94.1772   $94.18   $94.20   $94.20  
  7 purchase totals = $806.76       $881.2701   $881.26   $881.25   $881.30