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 $249.91   10.649%   $276.5229   $276.52   $276.50   $276.50  
  Purchase #2 $180.88   3.760%   $187.6811   $187.68   $187.70   $187.70  
  Purchase #3 $226.79   9.323%   $247.9336   $247.93   $247.95   $247.90  
  Purchase #4 $234.35   3.851%   $243.3748   $243.37   $243.35   $243.40  
  Purchase #5 $61.71   4.156%   $64.2747   $64.27   $64.25   $64.30  
  Purchase #6 $24.43   3.180%   $25.2069   $25.21   $25.20   $25.20  
  Purchase #7 $11.55   6.353%   $12.2838   $12.28   $12.30   $12.30  
  7 purchase totals = $989.62       $1,057.2778   $1,057.26   $1,057.25   $1,057.30