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 $138.44   3.351%   $143.0791   $143.08   $143.10   $143.10  
  Purchase #2 $123.48   5.699%   $130.5171   $130.52   $130.50   $130.50  
  Purchase #3 $104.93   12.510%   $118.0567   $118.06   $118.05   $118.10  
  Purchase #4 $168.74   5.843%   $178.5995   $178.60   $178.60   $178.60  
  Purchase #5 $89.69   6.471%   $95.4938   $95.49   $95.50   $95.50  
  Purchase #6 $133.86   4.420%   $139.7766   $139.78   $139.80   $139.80  
  Purchase #7 $86.70   12.732%   $97.7386   $97.74   $97.75   $97.70  
  7 purchase totals = $845.84       $903.2614   $903.27   $903.30   $903.30