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 $67.54   12.178%   $75.765   $75.77   $75.75   $75.80  
  Purchase #2 $60.86   6.636%   $64.8987   $64.90   $64.90   $64.90  
  Purchase #3 $58.76   6.176%   $62.389   $62.39   $62.40   $62.40  
  Purchase #4 $107.67   9.648%   $118.058   $118.06   $118.05   $118.10  
  Purchase #5 $183.40   10.361%   $202.4021   $202.40   $202.40   $202.40  
  Purchase #6 $104.31   11.594%   $116.4037   $116.40   $116.40   $116.40  
  Purchase #7 $59.93   12.424%   $67.3757   $67.38   $67.40   $67.40  
  7 purchase totals = $642.47       $707.2922   $707.30   $707.30   $707.40