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 $246.65   13.554%   $280.0809   $280.08   $280.10   $280.10  
  Purchase #2 $126.91   9.508%   $138.9766   $138.98   $139.00   $139.00  
  Purchase #3 $233.77   10.149%   $257.4953   $257.50   $257.50   $257.50  
  Purchase #4 $135.40   4.229%   $141.1261   $141.13   $141.15   $141.10  
  Purchase #5 $129.64   3.802%   $134.5689   $134.57   $134.55   $134.60  
  Purchase #6 $149.27   4.559%   $156.0752   $156.08   $156.10   $156.10  
  Purchase #7 $212.78   4.503%   $222.3615   $222.36   $222.35   $222.40  
  7 purchase totals = $1,234.42       $1,330.6845   $1,330.70   $1,330.75   $1,330.80