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 $85.42   12.139%   $95.7891   $95.79   $95.80   $95.80  
  Purchase #2 $167.91   12.534%   $188.9558   $188.96   $188.95   $189.00  
  Purchase #3 $248.50   11.374%   $276.7644   $276.76   $276.75   $276.80  
  Purchase #4 $249.53   9.465%   $273.148   $273.15   $273.15   $273.10  
  Purchase #5 $86.78   7.228%   $93.0525   $93.05   $93.05   $93.10  
  Purchase #6 $39.75   9.610%   $43.57   $43.57   $43.55   $43.60  
  Purchase #7 $100.85   5.156%   $106.0498   $106.05   $106.05   $106.00  
  7 purchase totals = $978.74       $1,077.3296   $1,077.33   $1,077.30   $1,077.40