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 $45.88   10.766%   $50.8194   $50.82   $50.80   $50.80  
  Purchase #2 $126.81   5.640%   $133.9621   $133.96   $133.95   $134.00  
  Purchase #3 $38.31   11.416%   $42.6835   $42.68   $42.70   $42.70  
  Purchase #4 $162.52   3.110%   $167.5744   $167.57   $167.55   $167.60  
  Purchase #5 $118.48   7.444%   $127.2997   $127.30   $127.30   $127.30  
  Purchase #6 $209.39   8.384%   $226.9453   $226.95   $226.95   $226.90  
  Purchase #7 $75.62   11.227%   $84.1099   $84.11   $84.10   $84.10  
  7 purchase totals = $777.01       $833.3943   $833.39   $833.35   $833.40