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 $66.54   12.497%   $74.8555   $74.86   $74.85   $74.90  
  Purchase #2 $114.41   6.325%   $121.6464   $121.65   $121.65   $121.60  
  Purchase #3 $184.91   13.282%   $209.4697   $209.47   $209.45   $209.50  
  Purchase #4 $68.49   6.327%   $72.8234   $72.82   $72.80   $72.80  
  Purchase #5 $155.84   12.323%   $175.0442   $175.04   $175.05   $175.00  
  Purchase #6 $75.83   9.456%   $83.0005   $83.00   $83.00   $83.00  
  Purchase #7 $9.28   8.343%   $10.0542   $10.05   $10.05   $10.10  
  7 purchase totals = $675.30       $746.8939   $746.89   $746.85   $746.90