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 $227.49   6.823%   $243.0116   $243.01   $243.00   $243.00  
  Purchase #2 $134.33   4.222%   $140.0014   $140.00   $140.00   $140.00  
  Purchase #3 $65.52   11.318%   $72.9356   $72.94   $72.95   $72.90  
  Purchase #4 $196.99   4.639%   $206.1284   $206.13   $206.15   $206.10  
  Purchase #5 $113.93   12.435%   $128.0972   $128.10   $128.10   $128.10  
  Purchase #6 $144.50   4.802%   $151.4389   $151.44   $151.45   $151.40  
  Purchase #7 $51.25   4.844%   $53.7326   $53.73   $53.75   $53.70  
  7 purchase totals = $934.01       $995.3457   $995.35   $995.40   $995.20