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 $50.56   11.156%   $56.2005   $56.20   $56.20   $56.20  
  Purchase #2 $98.29   8.508%   $106.6525   $106.65   $106.65   $106.70  
  Purchase #3 $127.93   8.118%   $138.3154   $138.32   $138.30   $138.30  
  Purchase #4 $84.78   13.219%   $95.9871   $95.99   $96.00   $96.00  
  Purchase #5 $54.66   10.591%   $60.449   $60.45   $60.45   $60.40  
  Purchase #6 $61.48   7.770%   $66.257   $66.26   $66.25   $66.30  
  Purchase #7 $74.69   10.480%   $82.5175   $82.52   $82.50   $82.50  
  7 purchase totals = $552.39       $606.3790   $606.39   $606.35   $606.40