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 $243.84   11.241%   $271.2501   $271.25   $271.25   $271.30  
  Purchase #2 $212.47   12.793%   $239.6513   $239.65   $239.65   $239.70  
  Purchase #3 $187.89   3.281%   $194.0547   $194.05   $194.05   $194.10  
  Purchase #4 $218.84   12.825%   $246.9062   $246.91   $246.90   $246.90  
  Purchase #5 $126.87   4.273%   $132.2912   $132.29   $132.30   $132.30  
  Purchase #6 $62.44   5.648%   $65.9666   $65.97   $65.95   $66.00  
  Purchase #7 $147.27   12.192%   $165.2252   $165.23   $165.25   $165.20  
  7 purchase totals = $1,199.62       $1,315.3453   $1,315.35   $1,315.35   $1,315.50