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 $201.83   13.334%   $228.742   $228.74   $228.75   $228.70  
  Purchase #2 $60.96   12.866%   $68.8031   $68.80   $68.80   $68.80  
  Purchase #3 $25.45   4.122%   $26.499   $26.50   $26.50   $26.50  
  Purchase #4 $181.82   12.466%   $204.4857   $204.49   $204.50   $204.50  
  Purchase #5 $224.75   3.605%   $232.8522   $232.85   $232.85   $232.90  
  Purchase #6 $156.49   4.627%   $163.7308   $163.73   $163.75   $163.70  
  Purchase #7 $59.57   11.368%   $66.3419   $66.34   $66.35   $66.30  
  7 purchase totals = $910.87       $991.4547   $991.45   $991.50   $991.40