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 $67.58   8.473%   $73.3061   $73.31   $73.30   $73.30  
  Purchase #2 $37.36   7.117%   $40.0189   $40.02   $40.00   $40.00  
  Purchase #3 $120.49   11.479%   $134.321   $134.32   $134.30   $134.30  
  Purchase #4 $34.96   3.634%   $36.2304   $36.23   $36.25   $36.20  
  Purchase #5 $212.57   7.481%   $228.4724   $228.47   $228.45   $228.50  
  Purchase #6 $65.50   13.762%   $74.5141   $74.51   $74.50   $74.50  
  Purchase #7 $7.47   4.474%   $7.8042   $7.80   $7.80   $7.80  
  7 purchase totals = $545.93       $594.6671   $594.66   $594.60   $594.60