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 $16.99   11.790%   $18.9931   $18.99   $19.00   $19.00  
  Purchase #2 $239.61   3.424%   $247.8142   $247.81   $247.80   $247.80  
  Purchase #3 $185.74   9.762%   $203.8719   $203.87   $203.85   $203.90  
  Purchase #4 $157.86   3.791%   $163.8445   $163.84   $163.85   $163.80  
  Purchase #5 $180.44   4.303%   $188.2043   $188.20   $188.20   $188.20  
  Purchase #6 $28.29   10.180%   $31.1699   $31.17   $31.15   $31.20  
  Purchase #7 $36.97   8.133%   $39.9768   $39.98   $40.00   $40.00  
  7 purchase totals = $845.90       $893.8747   $893.86   $893.85   $893.90