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 $118.95   13.663%   $135.2021   $135.20   $135.20   $135.20  
  Purchase #2 $209.78   6.557%   $223.5353   $223.54   $223.55   $223.50  
  Purchase #3 $246.56   9.566%   $270.1459   $270.15   $270.15   $270.10  
  Purchase #4 $50.98   12.155%   $57.1766   $57.18   $57.20   $57.20  
  Purchase #5 $38.47   13.701%   $43.7408   $43.74   $43.75   $43.70  
  Purchase #6 $55.57   5.458%   $58.603   $58.60   $58.60   $58.60  
  Purchase #7 $202.31   3.797%   $209.9917   $209.99   $210.00   $210.00  
  7 purchase totals = $922.62       $998.3954   $998.40   $998.45   $998.30