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 $52.39   13.196%   $59.3034   $59.30   $59.30   $59.30  
  Purchase #2 $231.48   8.356%   $250.8225   $250.82   $250.80   $250.80  
  Purchase #3 $156.75   13.351%   $177.6777   $177.68   $177.70   $177.70  
  Purchase #4 $247.32   7.489%   $265.8418   $265.84   $265.85   $265.80  
  Purchase #5 $115.57   13.110%   $130.7212   $130.72   $130.70   $130.70  
  Purchase #6 $62.70   11.202%   $69.7237   $69.72   $69.70   $69.70  
  Purchase #7 $119.29   13.342%   $135.2057   $135.21   $135.20   $135.20  
  7 purchase totals = $985.50       $1,089.2960   $1,089.29   $1,089.25   $1,089.20