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 $248.84   9.801%   $273.2288   $273.23   $273.25   $273.20  
  Purchase #2 $58.62   5.176%   $61.6542   $61.65   $61.65   $61.70  
  Purchase #3 $63.82   5.759%   $67.4954   $67.50   $67.50   $67.50  
  Purchase #4 $223.68   4.308%   $233.3161   $233.32   $233.30   $233.30  
  Purchase #5 $169.36   10.565%   $187.2529   $187.25   $187.25   $187.30  
  Purchase #6 $235.67   13.525%   $267.5444   $267.54   $267.55   $267.50  
  Purchase #7 $174.44   11.119%   $193.836   $193.84   $193.85   $193.80  
  7 purchase totals = $1,174.43       $1,284.3278   $1,284.33   $1,284.35   $1,284.30