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 $236.38   9.570%   $259.0016   $259.00   $259.00   $259.00  
  Purchase #2 $179.27   11.373%   $199.6584   $199.66   $199.65   $199.70  
  Purchase #3 $232.98   3.436%   $240.9852   $240.99   $241.00   $241.00  
  Purchase #4 $98.52   13.332%   $111.6547   $111.65   $111.65   $111.70  
  Purchase #5 $235.63   9.159%   $257.2114   $257.21   $257.20   $257.20  
  Purchase #6 $225.80   11.251%   $251.2048   $251.20   $251.20   $251.20  
  Purchase #7 $55.27   6.489%   $58.8565   $58.86   $58.85   $58.90  
  7 purchase totals = $1,263.85       $1,378.5726   $1,378.57   $1,378.55   $1,378.70