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 $170.77   4.555%   $178.5486   $178.55   $178.55   $178.50  
  Purchase #2 $73.60   8.584%   $79.9178   $79.92   $79.90   $79.90  
  Purchase #3 $139.80   5.184%   $147.0472   $147.05   $147.05   $147.00  
  Purchase #4 $74.40   5.471%   $78.4704   $78.47   $78.45   $78.50  
  Purchase #5 $76.30   10.119%   $84.0208   $84.02   $84.00   $84.00  
  Purchase #6 $145.38   10.760%   $161.0229   $161.02   $161.00   $161.00  
  Purchase #7 $153.84   4.489%   $160.7459   $160.75   $160.75   $160.70  
  7 purchase totals = $834.09       $889.7736   $889.78   $889.70   $889.60