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 $249.43   3.485%   $258.1226   $258.12   $258.10   $258.10  
  Purchase #2 $40.34   3.261%   $41.6555   $41.66   $41.65   $41.70  
  Purchase #3 $159.67   10.832%   $176.9655   $176.97   $176.95   $177.00  
  Purchase #4 $51.84   5.337%   $54.6067   $54.61   $54.60   $54.60  
  Purchase #5 $88.54   11.334%   $98.5751   $98.58   $98.60   $98.60  
  Purchase #6 $71.78   10.828%   $79.5523   $79.55   $79.55   $79.60  
  Purchase #7 $93.29   3.150%   $96.2286   $96.23   $96.25   $96.20  
  7 purchase totals = $754.89       $805.7063   $805.72   $805.70   $805.80