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 $7.36   7.509%   $7.9127   $7.91   $7.90   $7.90  
  Purchase #2 $7.93   12.222%   $8.8992   $8.90   $8.90   $8.90  
  Purchase #3 $104.61   12.548%   $117.7365   $117.74   $117.75   $117.70  
  Purchase #4 $225.81   11.799%   $252.4533   $252.45   $252.45   $252.50  
  Purchase #5 $188.38   12.325%   $211.5978   $211.60   $211.60   $211.60  
  Purchase #6 $228.94   13.648%   $260.1857   $260.19   $260.20   $260.20  
  Purchase #7 $249.37   12.254%   $279.9278   $279.93   $279.95   $279.90  
  7 purchase totals = $1,012.40       $1,138.7130   $1,138.72   $1,138.75   $1,138.70