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 $227.69   10.282%   $251.1011   $251.10   $251.10   $251.10  
  Purchase #2 $30.87   4.756%   $32.3382   $32.34   $32.35   $32.30  
  Purchase #3 $17.55   10.362%   $19.3685   $19.37   $19.35   $19.40  
  Purchase #4 $43.59   9.622%   $47.7842   $47.78   $47.80   $47.80  
  Purchase #5 $155.81   10.406%   $172.0236   $172.02   $172.00   $172.00  
  Purchase #6 $77.81   11.827%   $87.0126   $87.01   $87.00   $87.00  
  Purchase #7 $57.27   6.338%   $60.8998   $60.90   $60.90   $60.90  
  7 purchase totals = $610.59       $670.5280   $670.52   $670.50   $670.50