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 $30.61   13.427%   $34.72   $34.72   $34.70   $34.70  
  Purchase #2 $85.67   6.647%   $91.3645   $91.36   $91.35   $91.40  
  Purchase #3 $113.87   3.222%   $117.5389   $117.54   $117.55   $117.50  
  Purchase #4 $179.95   12.151%   $201.8157   $201.82   $201.80   $201.80  
  Purchase #5 $226.77   11.527%   $252.9098   $252.91   $252.90   $252.90  
  Purchase #6 $16.94   11.103%   $18.8208   $18.82   $18.80   $18.80  
  Purchase #7 $78.99   7.107%   $84.6038   $84.60   $84.60   $84.60  
  7 purchase totals = $732.80       $801.7735   $801.77   $801.70   $801.70