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 $80.33   12.389%   $90.2821   $90.28   $90.30   $90.30  
  Purchase #2 $99.70   8.410%   $108.0848   $108.08   $108.10   $108.10  
  Purchase #3 $146.67   3.785%   $152.2215   $152.22   $152.20   $152.20  
  Purchase #4 $243.89   3.223%   $251.7506   $251.75   $251.75   $251.80  
  Purchase #5 $160.62   8.334%   $174.0061   $174.01   $174.00   $174.00  
  Purchase #6 $34.65   12.834%   $39.097   $39.10   $39.10   $39.10  
  Purchase #7 $79.50   4.221%   $82.8557   $82.86   $82.85   $82.90  
  7 purchase totals = $845.36       $898.2978   $898.30   $898.30   $898.40