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 $74.69   11.370%   $83.1823   $83.18   $83.20   $83.20  
  Purchase #2 $39.98   4.809%   $41.9026   $41.90   $41.90   $41.90  
  Purchase #3 $165.79   11.398%   $184.6867   $184.69   $184.70   $184.70  
  Purchase #4 $118.80   12.662%   $133.8425   $133.84   $133.85   $133.80  
  Purchase #5 $240.51   10.724%   $266.3023   $266.30   $266.30   $266.30  
  Purchase #6 $189.73   5.471%   $200.1101   $200.11   $200.10   $200.10  
  Purchase #7 $81.43   10.810%   $90.2326   $90.23   $90.25   $90.20  
  7 purchase totals = $910.93       $1,000.2591   $1,000.25   $1,000.30   $1,000.20