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 $160.67   3.309%   $165.9866   $165.99   $166.00   $166.00  
  Purchase #2 $37.55   11.638%   $41.9201   $41.92   $41.90   $41.90  
  Purchase #3 $27.96   5.193%   $29.412   $29.41   $29.40   $29.40  
  Purchase #4 $246.99   3.190%   $254.869   $254.87   $254.85   $254.90  
  Purchase #5 $86.75   12.181%   $97.317   $97.32   $97.30   $97.30  
  Purchase #6 $67.41   11.818%   $75.3765   $75.38   $75.40   $75.40  
  Purchase #7 $155.78   3.366%   $161.0236   $161.02   $161.00   $161.00  
  7 purchase totals = $783.11       $825.9048   $825.91   $825.85   $825.90