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 $30.78   3.414%   $31.8308   $31.83   $31.85   $31.80  
  Purchase #2 $31.80   8.474%   $34.4947   $34.49   $34.50   $34.50  
  Purchase #3 $203.29   8.295%   $220.1529   $220.15   $220.15   $220.20  
  Purchase #4 $142.94   3.430%   $147.8428   $147.84   $147.85   $147.80  
  Purchase #5 $58.56   11.721%   $65.4238   $65.42   $65.40   $65.40  
  Purchase #6 $105.90   12.332%   $118.9596   $118.96   $118.95   $119.00  
  Purchase #7 $72.59   11.636%   $81.0366   $81.04   $81.05   $81.00  
  7 purchase totals = $645.86       $699.7412   $699.73   $699.75   $699.70