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 $241.68   13.344%   $273.9298   $273.93   $273.95   $273.90  
  Purchase #2 $41.85   4.397%   $43.6901   $43.69   $43.70   $43.70  
  Purchase #3 $148.86   7.675%   $160.285   $160.29   $160.30   $160.30  
  Purchase #4 $109.57   10.549%   $121.1285   $121.13   $121.15   $121.10  
  Purchase #5 $136.58   3.296%   $141.0817   $141.08   $141.10   $141.10  
  Purchase #6 $96.97   4.692%   $101.5198   $101.52   $101.50   $101.50  
  Purchase #7 $42.35   11.629%   $47.2749   $47.27   $47.25   $47.30  
  7 purchase totals = $817.86       $888.9098   $888.91   $888.95   $888.90