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 $7.40   5.732%   $7.8242   $7.82   $7.80   $7.80  
  Purchase #2 $38.76   8.257%   $41.9604   $41.96   $41.95   $42.00  
  Purchase #3 $129.57   12.136%   $145.2946   $145.29   $145.30   $145.30  
  Purchase #4 $220.77   11.570%   $246.3131   $246.31   $246.30   $246.30  
  Purchase #5 $171.37   4.500%   $179.0817   $179.08   $179.10   $179.10  
  Purchase #6 $48.43   9.454%   $53.0086   $53.01   $53.00   $53.00  
  Purchase #7 $52.94   3.489%   $54.7871   $54.79   $54.80   $54.80  
  7 purchase totals = $669.24       $728.2697   $728.26   $728.25   $728.30